diff --git a/src/Interfaces/Payment/PaymentAttemptFailedInterface.php b/src/Interfaces/Payment/PaymentAttemptFailedInterface.php
index ff252f1..e582f8b 100644
--- a/src/Interfaces/Payment/PaymentAttemptFailedInterface.php
+++ b/src/Interfaces/Payment/PaymentAttemptFailedInterface.php
@@ -4,6 +4,10 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * A failed payment attempt - it was authorized successfully, but the money did not move. Only reachable
+ * when payment retries are enabled; the payment may still succeed on a later attempt.
+ */
 interface PaymentAttemptFailedInterface extends PaymentFailureInterface
 {
 }
diff --git a/src/Interfaces/Payment/PaymentAuthorizationRequiredInterface.php b/src/Interfaces/Payment/PaymentAuthorizationRequiredInterface.php
index b71537f..8acb592 100644
--- a/src/Interfaces/Payment/PaymentAuthorizationRequiredInterface.php
+++ b/src/Interfaces/Payment/PaymentAuthorizationRequiredInterface.php
@@ -4,6 +4,10 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * A payment yet to be authorized. This is the first status for a payment that requires further actions to
+ * be authorized.
+ */
 interface PaymentAuthorizationRequiredInterface extends PaymentRetrievedInterface
 {
 }
diff --git a/src/Interfaces/Payment/PaymentAuthorizedInterface.php b/src/Interfaces/Payment/PaymentAuthorizedInterface.php
index 322b521..8c7e5e6 100644
--- a/src/Interfaces/Payment/PaymentAuthorizedInterface.php
+++ b/src/Interfaces/Payment/PaymentAuthorizedInterface.php
@@ -6,14 +6,22 @@
 
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ConfigurationInterface;
 
+/**
+ * An authorized payment - no more actions required, waiting for outcome.
+ */
 interface PaymentAuthorizedInterface extends PaymentRetrievedInterface
 {
     /**
+     * Information about the authorization flow the payment went through. Null if the payment transitioned
+     * immediately to authorized or failed.
+     *
      * @return ConfigurationInterface|null
      */
     public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
 
     /**
+     * The date and time that TrueLayer determined that the payment was ready to be credited.
+     *
      * @return \DateTimeInterface|null
      */
     public function getCreditableAt(): ?\DateTimeInterface;
diff --git a/src/Interfaces/Payment/PaymentAuthorizingInterface.php b/src/Interfaces/Payment/PaymentAuthorizingInterface.php
index 70339c8..1e50586 100644
--- a/src/Interfaces/Payment/PaymentAuthorizingInterface.php
+++ b/src/Interfaces/Payment/PaymentAuthorizingInterface.php
@@ -7,14 +7,22 @@
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ActionInterface;
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ConfigurationInterface;
 
+/**
+ * The payment's authorization_flow has started, but authorization has not completed yet.
+ */
 interface PaymentAuthorizingInterface extends PaymentRetrievedInterface
 {
     /**
+     * The next action the user must take in the authorization flow, such as selecting a provider or being
+     * redirected.
+     *
      * @return ActionInterface|null
      */
     public function getAuthorizationFlowNextAction(): ?ActionInterface;
 
     /**
+     * Information about the authorization flow the payment went through.
+     *
      * @return ConfigurationInterface|null
      */
     public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
diff --git a/src/Interfaces/Payment/PaymentCreatedInterface.php b/src/Interfaces/Payment/PaymentCreatedInterface.php
index a29af11..87490a5 100644
--- a/src/Interfaces/Payment/PaymentCreatedInterface.php
+++ b/src/Interfaces/Payment/PaymentCreatedInterface.php
@@ -12,29 +12,44 @@
 use TrueLayer\Interfaces\HppInterface;
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\AuthorizationFlowAuthorizingInterface;
 
+/**
+ * The newly created payment, as returned by the payment creation response.
+ */
 interface PaymentCreatedInterface extends ArrayableInterface
 {
     /**
+     * The unique ID of the payment.
+     *
      * @return string
      */
     public function getId(): string;
 
     /**
+     * A payment token in JWT format, with a limited scope that is authorised to be used with this single
+     * payment.
+     *
      * @return string
      */
     public function getResourceToken(): string;
 
     /**
+     * The unique ID of the user. If a user ID wasn't passed in the request body while creating the
+     * payment, TrueLayer generates one and returns it here.
+     *
      * @return string
      */
     public function getUserId(): string;
 
     /**
+     * The Hosted Payments Page URI, if a hosted_page was requested on creation.
+     *
      * @return string|null
      */
     public function getHostedPageUri(): ?string;
 
     /**
+     * Builds the Hosted Payments Page URL for this payment.
+     *
      * @return HppInterface
      */
     public function hostedPaymentsPage(): HppInterface;
@@ -54,6 +69,8 @@ public function hostedPaymentsPage(): HppInterface;
     public function startAuthorization(string $returnUri): AuthorizationFlowAuthorizingInterface;
 
     /**
+     * Starts building a request to manually start the authorization flow for this payment.
+     *
      * @throws InvalidArgumentException
      *
      * @return StartAuthorizationFlowRequestInterface
@@ -61,6 +78,9 @@ public function startAuthorization(string $returnUri): AuthorizationFlowAuthoriz
     public function authorizationFlow(): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Retrieves the full payment details, equivalent to calling `getPayment()` on the client with this
+     * payment's ID.
+     *
      * @throws ApiRequestJsonSerializationException
      * @throws ApiResponseUnsuccessfulException
      *
diff --git a/src/Interfaces/Payment/PaymentExecutedInterface.php b/src/Interfaces/Payment/PaymentExecutedInterface.php
index 6905a42..2bf0139 100644
--- a/src/Interfaces/Payment/PaymentExecutedInterface.php
+++ b/src/Interfaces/Payment/PaymentExecutedInterface.php
@@ -6,19 +6,29 @@
 
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ConfigurationInterface;
 
+/**
+ * An executed payment - the bank has accepted the payment.
+ */
 interface PaymentExecutedInterface extends PaymentRetrievedInterface
 {
     /**
+     * The date and time the payment executed at.
+     *
      * @return \DateTimeInterface
      */
     public function getExecutedAt(): \DateTimeInterface;
 
     /**
+     * Information about the authorization flow the payment went through. Null if the payment transitioned
+     * immediately to authorized or failed.
+     *
      * @return ConfigurationInterface|null
      */
     public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
 
     /**
+     * The date and time that TrueLayer determined that the payment was ready to be credited.
+     *
      * @return \DateTimeInterface|null
      */
     public function getCreditableAt(): ?\DateTimeInterface;
diff --git a/src/Interfaces/Payment/PaymentFailedAttemptInterface.php b/src/Interfaces/Payment/PaymentFailedAttemptInterface.php
index 5e474db..b6fafe4 100644
--- a/src/Interfaces/Payment/PaymentFailedAttemptInterface.php
+++ b/src/Interfaces/Payment/PaymentFailedAttemptInterface.php
@@ -6,19 +6,29 @@
 
 use TrueLayer\Interfaces\ArrayableInterface;
 
+/**
+ * A failed payment attempt, recorded when payment retries are enabled. Contains details about when the
+ * attempt failed and why.
+ */
 interface PaymentFailedAttemptInterface extends ArrayableInterface
 {
     /**
+     * The sequential number of this payment attempt.
+     *
      * @return int
      */
     public function getAttemptNumber(): int;
 
     /**
+     * The date and time this payment attempt failed.
+     *
      * @return \DateTimeInterface
      */
     public function getFailedAt(): \DateTimeInterface;
 
     /**
+     * The reason this attempt failed. Handle unexpected values gracefully as an unknown failure.
+     *
      * @return string
      */
     public function getFailureReason(): string;
diff --git a/src/Interfaces/Payment/PaymentFailedInterface.php b/src/Interfaces/Payment/PaymentFailedInterface.php
index 405d3a3..e3adf98 100644
--- a/src/Interfaces/Payment/PaymentFailedInterface.php
+++ b/src/Interfaces/Payment/PaymentFailedInterface.php
@@ -4,24 +4,36 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * A failed payment - it was authorized successfully, but the money did not move.
+ */
 interface PaymentFailedInterface extends PaymentFailureInterface
 {
     /**
+     * The date and time that TrueLayer determined that the payment was ready to be credited.
+     *
      * @return \DateTimeInterface|null
      */
     public function getCreditableAt(): ?\DateTimeInterface;
 
     /**
+     * The date and time at which TrueLayer determined that the payment's settlement was stalled, based on
+     * the client's chosen delay or the default one.
+     *
      * @return \DateTimeInterface|null
      */
     public function getSettlementStalledAt(): ?\DateTimeInterface;
 
     /**
+     * The date and time the payment was reversed, if it was.
+     *
      * @return \DateTimeInterface|null
      */
     public function getReversedAt(): ?\DateTimeInterface;
 
     /**
+     * The reason for the payment reversal, one of `corrected`, `failed` or `disputed`.
+     *
      * @return string|null
      */
     public function getReversalReason(): ?string;
diff --git a/src/Interfaces/Payment/PaymentFailureInterface.php b/src/Interfaces/Payment/PaymentFailureInterface.php
index 902c10c..91a8260 100644
--- a/src/Interfaces/Payment/PaymentFailureInterface.php
+++ b/src/Interfaces/Payment/PaymentFailureInterface.php
@@ -6,24 +6,39 @@
 
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ConfigurationInterface;
 
+/**
+ * Fields shared by a payment that was authorized successfully, but the money did not move (either
+ * permanently, via `PaymentFailedInterface`, or for a single retried attempt, via
+ * `PaymentAttemptFailedInterface`).
+ */
 interface PaymentFailureInterface extends PaymentRetrievedInterface
 {
     /**
+     * The date and time the payment was marked as failed.
+     *
      * @return \DateTimeInterface
      */
     public function getFailedAt(): \DateTimeInterface;
 
     /**
+     * The status the payment was in when it failed, one of `authorization_required`, `authorizing`,
+     * `authorized` or `executed`.
+     *
      * @return string
      */
     public function getFailureStage(): string;
 
     /**
+     * The reason the payment failed. Handle unexpected values gracefully as an unknown failure.
+     *
      * @return string|null
      */
     public function getFailureReason(): ?string;
 
     /**
+     * Information about the authorization flow the payment went through. Null if the payment transitioned
+     * immediately to authorized or failed.
+     *
      * @return ConfigurationInterface|null
      */
     public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
diff --git a/src/Interfaces/Payment/PaymentHostedPageInterface.php b/src/Interfaces/Payment/PaymentHostedPageInterface.php
index 5284a99..5c44d21 100644
--- a/src/Interfaces/Payment/PaymentHostedPageInterface.php
+++ b/src/Interfaces/Payment/PaymentHostedPageInterface.php
@@ -4,6 +4,10 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * Configures an auto-constructed Hosted Payments Page URI, returned on the payment creation response.
+ * Cannot be provided if starting the authorization flow explicitly.
+ */
 interface PaymentHostedPageInterface
 {
     /**
@@ -12,6 +16,9 @@ interface PaymentHostedPageInterface
     public function getReturnUri(): ?string;
 
     /**
+     * The URI where the user will be redirected after the authorization flow has completed on the hosted
+     * page. Must be registered as an allowed redirect URI in the Console.
+     *
      * @param string $returnUri
      *
      * @return PaymentHostedPageInterface
@@ -24,6 +31,8 @@ public function returnUri(string $returnUri): PaymentHostedPageInterface;
     public function getCountryCode(): ?string;
 
     /**
+     * The country code of the user, used to determine which banks to show on the hosted page initially.
+     *
      * @param string $countryCode ISO 3166-1 alpha-2 country code
      *
      * @return PaymentHostedPageInterface
@@ -36,6 +45,9 @@ public function countryCode(string $countryCode): PaymentHostedPageInterface;
     public function getLanguageCode(): ?string;
 
     /**
+     * The language code of the user, used to determine which language to show on the hosted page,
+     * overriding the browser's locale.
+     *
      * @param string $languageCode ISO 639-1 language code
      *
      * @return PaymentHostedPageInterface
@@ -48,6 +60,9 @@ public function languageCode(string $languageCode): PaymentHostedPageInterface;
     public function getMaxWaitForResult(): ?int;
 
     /**
+     * The maximum time to wait for a result from the hosted page after the user has completed the
+     * authorization flow.
+     *
      * @param int $maxWaitForResult Seconds to wait for a result, between 0 and 60
      *
      * @return PaymentHostedPageInterface
diff --git a/src/Interfaces/Payment/PaymentRequestInterface.php b/src/Interfaces/Payment/PaymentRequestInterface.php
index db28bf7..be46f32 100644
--- a/src/Interfaces/Payment/PaymentRequestInterface.php
+++ b/src/Interfaces/Payment/PaymentRequestInterface.php
@@ -9,9 +9,14 @@
 use TrueLayer\Interfaces\RequestOptionsInterface;
 use TrueLayer\Interfaces\UserInterface;
 
+/**
+ * Builds a payment creation request.
+ */
 interface PaymentRequestInterface extends HasAttributesInterface
 {
     /**
+     * A 'cent' value representing the amount, e.g. 100 = 1 GBP or 100 = 1 EUR.
+     *
      * @param int $amount
      *
      * @return PaymentRequestInterface
@@ -19,6 +24,8 @@ interface PaymentRequestInterface extends HasAttributesInterface
     public function amountInMinor(int $amount): PaymentRequestInterface;
 
     /**
+     * The three-letter ISO 4217 currency code the payment should be made in.
+     *
      * @param string $currency
      *
      * @return PaymentRequestInterface
@@ -26,6 +33,9 @@ public function amountInMinor(int $amount): PaymentRequestInterface;
     public function currency(string $currency): PaymentRequestInterface;
 
     /**
+     * Optional custom key-value data to attach to the payment. Limited to 10 pairs, each key up to 40
+     * characters and each value up to 500 characters.
+     *
      * @param array<string, string> $metadata
      *
      * @return PaymentRequestInterface
@@ -33,6 +43,8 @@ public function currency(string $currency): PaymentRequestInterface;
     public function metadata(array $metadata): PaymentRequestInterface;
 
     /**
+     * The payment method to use, such as a bank transfer.
+     *
      * @param PaymentMethodInterface $paymentMethod
      *
      * @return PaymentRequestInterface
@@ -40,6 +52,9 @@ public function metadata(array $metadata): PaymentRequestInterface;
     public function paymentMethod(PaymentMethodInterface $paymentMethod): PaymentRequestInterface;
 
     /**
+     * Optional configuration for risk assessment and the `payment_creditable` webhook. Contact TrueLayer
+     * before sending this field.
+     *
      * @param PaymentRiskAssessmentInterface|null $riskAssessment
      *
      * @return PaymentRiskAssessmentInterface
@@ -47,6 +62,9 @@ public function paymentMethod(PaymentMethodInterface $paymentMethod): PaymentReq
     public function riskAssessment(?PaymentRiskAssessmentInterface $riskAssessment): PaymentRiskAssessmentInterface;
 
     /**
+     * Details of the end user making the payment. Not required for payments made over a mandate, or when
+     * using your own PISP licence.
+     *
      * @param UserInterface $user
      *
      * @return PaymentRequestInterface
@@ -54,6 +72,10 @@ public function riskAssessment(?PaymentRiskAssessmentInterface $riskAssessment):
     public function user(UserInterface $user): PaymentRequestInterface;
 
     /**
+     * Requests a hosted page URI as part of payment creation, so you don't have to build the Hosted
+     * Payments Page URL yourself afterwards. Cannot be combined with starting the authorization flow
+     * explicitly.
+     *
      * @param PaymentHostedPageInterface|null $hostedPage
      *
      * @return PaymentHostedPageInterface
@@ -68,6 +90,8 @@ public function hostedPage(?PaymentHostedPageInterface $hostedPage = null): Paym
     public function requestOptions(RequestOptionsInterface $requestOptions): PaymentRequestInterface;
 
     /**
+     * Send the request to create the payment.
+     *
      * @return PaymentCreatedInterface
      */
     public function create(): PaymentCreatedInterface;
diff --git a/src/Interfaces/Payment/PaymentRetrievedInterface.php b/src/Interfaces/Payment/PaymentRetrievedInterface.php
index 2d208e7..abe814a 100644
--- a/src/Interfaces/Payment/PaymentRetrievedInterface.php
+++ b/src/Interfaces/Payment/PaymentRetrievedInterface.php
@@ -7,24 +7,35 @@
 use TrueLayer\Interfaces\ArrayableInterface;
 use TrueLayer\Interfaces\PaymentMethod\PaymentMethodInterface;
 
+/**
+ * Fields and helpers shared by a payment resource regardless of its current status.
+ */
 interface PaymentRetrievedInterface extends ArrayableInterface
 {
     /**
+     * The unique ID for the payment.
+     *
      * @return string
      */
     public function getId(): string;
 
     /**
+     * A 'cent' value representing the amount, e.g. 100 = 1 GBP or 100 = 1 EUR.
+     *
      * @return int
      */
     public function getAmountInMinor(): int;
 
     /**
+     * The three-letter ISO 4217 currency code the payment was made in.
+     *
      * @return string
      */
     public function getCurrency(): string;
 
     /**
+     * Custom key-value data attached to the payment.
+     *
      * @return array<string, string>
      */
     public function getMetadata(): array;
@@ -35,21 +46,29 @@ public function getMetadata(): array;
     public function getPaymentMethod(): PaymentMethodInterface;
 
     /**
+     * The unique ID of the user who made the payment.
+     *
      * @return string
      */
     public function getUserId(): string;
 
     /**
+     * The date and time the payment was created at.
+     *
      * @return \DateTimeInterface
      */
     public function getCreatedAt(): \DateTimeInterface;
 
     /**
+     * The current status of the payment.
+     *
      * @return string
      */
     public function getStatus(): string;
 
     /**
+     * The list of failed payment attempts, if any. Only populated when payment retries are enabled.
+     *
      * @return PaymentFailedAttemptInterface[]
      */
     public function getFailedAttempts(): array;
@@ -65,11 +84,15 @@ public function isAuthorizationRequired(): bool;
     public function isAuthorizing(): bool;
 
     /**
+     * Will also return false when the payment has progressed to executed, failed or settled states.
+     *
      * @return bool
      */
     public function isAuthorized(): bool;
 
     /**
+     * Will also return false when the payment has progressed to failed or settled states.
+     *
      * @return bool
      */
     public function isExecuted(): bool;
@@ -80,6 +103,8 @@ public function isExecuted(): bool;
     public function isFailed(): bool;
 
     /**
+     * Only true if payment retries are enabled and this attempt failed.
+     *
      * @return bool
      */
     public function isAttemptFailed(): bool;
@@ -90,6 +115,9 @@ public function isAttemptFailed(): bool;
     public function isSettled(): bool;
 
     /**
+     * Cancels the payment, as long as it hasn't been authorized yet, and returns a fresh version of the
+     * retrieved payment.
+     *
      * @return PaymentRetrievedInterface
      */
     public function cancel(): PaymentRetrievedInterface;
diff --git a/src/Interfaces/Payment/PaymentRiskAssessmentInterface.php b/src/Interfaces/Payment/PaymentRiskAssessmentInterface.php
index 69c0fb2..bfe5e2a 100644
--- a/src/Interfaces/Payment/PaymentRiskAssessmentInterface.php
+++ b/src/Interfaces/Payment/PaymentRiskAssessmentInterface.php
@@ -4,6 +4,9 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * Optional configuration for risk assessment and the `payment_creditable` webhook.
+ */
 interface PaymentRiskAssessmentInterface
 {
     /**
@@ -12,6 +15,8 @@ interface PaymentRiskAssessmentInterface
     public function getSegment(): ?string;
 
     /**
+     * The risk segment of this payment. Contact TrueLayer before sending this field.
+     *
      * @param string $segment
      *
      * @return PaymentRiskAssessmentInterface
diff --git a/src/Interfaces/Payment/PaymentSettledInterface.php b/src/Interfaces/Payment/PaymentSettledInterface.php
index 5b0ef11..3a03ec5 100644
--- a/src/Interfaces/Payment/PaymentSettledInterface.php
+++ b/src/Interfaces/Payment/PaymentSettledInterface.php
@@ -10,34 +10,51 @@
 use TrueLayer\Exceptions\SignerException;
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\ConfigurationInterface;
 
+/**
+ * A settled payment, i.e. the funds have settled into the beneficiary account. Only reachable for payments
+ * into merchant accounts.
+ */
 interface PaymentSettledInterface extends PaymentRetrievedInterface
 {
     /**
+     * Information about the source of funds, if available from the remitter's bank.
+     *
      * @return PaymentSourceInterface
      */
     public function getPaymentSource(): PaymentSourceInterface;
 
     /**
+     * The date and time the payment settled.
+     *
      * @return \DateTimeInterface
      */
     public function getSettledAt(): \DateTimeInterface;
 
     /**
+     * The date and time the payment executed.
+     *
      * @return \DateTimeInterface
      */
     public function getExecutedAt(): \DateTimeInterface;
 
     /**
+     * The date and time that TrueLayer determined that the payment was ready to be credited.
+     *
      * @return \DateTimeInterface|null
      */
     public function getCreditableAt(): ?\DateTimeInterface;
 
     /**
+     * Information about the authorization flow the payment went through. Null if the payment transitioned
+     * immediately to authorized or failed.
+     *
      * @return ConfigurationInterface|null
      */
     public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
 
     /**
+     * Starts building a refund request for this payment.
+     *
      * @throws InvalidArgumentException
      *
      * @return RefundRequestInterface
@@ -45,6 +62,8 @@ public function getAuthorizationFlowConfig(): ?ConfigurationInterface;
     public function refund(): RefundRequestInterface;
 
     /**
+     * Retrieves a single refund created from this payment.
+     *
      * @param string $refundId
      *
      * @throws ApiResponseUnsuccessfulException
@@ -57,6 +76,8 @@ public function refund(): RefundRequestInterface;
     public function getRefund(string $refundId): RefundRetrievedInterface;
 
     /**
+     * Retrieves all refunds created from this payment.
+     *
      * @throws ApiResponseUnsuccessfulException
      * @throws InvalidArgumentException
      * @throws SignerException
diff --git a/src/Interfaces/Payment/PaymentSourceInterface.php b/src/Interfaces/Payment/PaymentSourceInterface.php
index 8d828b5..cc735a3 100644
--- a/src/Interfaces/Payment/PaymentSourceInterface.php
+++ b/src/Interfaces/Payment/PaymentSourceInterface.php
@@ -7,6 +7,10 @@
 use TrueLayer\Interfaces\AccountIdentifier\AccountIdentifierInterface;
 use TrueLayer\Interfaces\ArrayableInterface;
 
+/**
+ * Information about the source of funds, if available from the remitter's bank. Account identifiers will
+ * be those provided by the bank.
+ */
 interface PaymentSourceInterface extends ArrayableInterface
 {
     /**
diff --git a/src/Interfaces/Payment/RefundAuthorizedInterface.php b/src/Interfaces/Payment/RefundAuthorizedInterface.php
index be504b3..1f80d12 100644
--- a/src/Interfaces/Payment/RefundAuthorizedInterface.php
+++ b/src/Interfaces/Payment/RefundAuthorizedInterface.php
@@ -4,6 +4,9 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * An authorized refund - no more actions required, waiting for outcome.
+ */
 interface RefundAuthorizedInterface extends RefundRetrievedInterface
 {
 }
diff --git a/src/Interfaces/Payment/RefundCreatedInterface.php b/src/Interfaces/Payment/RefundCreatedInterface.php
index 9a9f572..59beedc 100644
--- a/src/Interfaces/Payment/RefundCreatedInterface.php
+++ b/src/Interfaces/Payment/RefundCreatedInterface.php
@@ -6,9 +6,14 @@
 
 use TrueLayer\Interfaces\HasAttributesInterface;
 
+/**
+ * The newly created refund, as returned by the refund creation response.
+ */
 interface RefundCreatedInterface extends HasAttributesInterface
 {
     /**
+     * The unique ID of the refund.
+     *
      * @return string
      */
     public function getId(): string;
diff --git a/src/Interfaces/Payment/RefundExecutedInterface.php b/src/Interfaces/Payment/RefundExecutedInterface.php
index 27a52ef..5f31d2c 100644
--- a/src/Interfaces/Payment/RefundExecutedInterface.php
+++ b/src/Interfaces/Payment/RefundExecutedInterface.php
@@ -4,9 +4,14 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * An executed refund.
+ */
 interface RefundExecutedInterface extends RefundRetrievedInterface
 {
     /**
+     * The date and time the refund executed.
+     *
      * @return \DateTimeInterface
      */
     public function getExecutedAt(): \DateTimeInterface;
diff --git a/src/Interfaces/Payment/RefundFailedInterface.php b/src/Interfaces/Payment/RefundFailedInterface.php
index 00702be..3331606 100644
--- a/src/Interfaces/Payment/RefundFailedInterface.php
+++ b/src/Interfaces/Payment/RefundFailedInterface.php
@@ -4,14 +4,21 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * A failed refund.
+ */
 interface RefundFailedInterface extends RefundRetrievedInterface
 {
     /**
+     * The date and time the refund failed.
+     *
      * @return \DateTimeInterface
      */
     public function getFailedAt(): \DateTimeInterface;
 
     /**
+     * The reason the refund failed. Handle unexpected values gracefully as an unknown failure.
+     *
      * @return string|null
      */
     public function getFailureReason(): ?string;
diff --git a/src/Interfaces/Payment/RefundPendingInterface.php b/src/Interfaces/Payment/RefundPendingInterface.php
index 13c89d7..70fe35b 100644
--- a/src/Interfaces/Payment/RefundPendingInterface.php
+++ b/src/Interfaces/Payment/RefundPendingInterface.php
@@ -4,6 +4,9 @@
 
 namespace TrueLayer\Interfaces\Payment;
 
+/**
+ * A refund yet to be authorized.
+ */
 interface RefundPendingInterface extends RefundRetrievedInterface
 {
 }
diff --git a/src/Interfaces/Payment/RefundRequestInterface.php b/src/Interfaces/Payment/RefundRequestInterface.php
index ba280ea..a985d58 100644
--- a/src/Interfaces/Payment/RefundRequestInterface.php
+++ b/src/Interfaces/Payment/RefundRequestInterface.php
@@ -7,9 +7,14 @@
 use TrueLayer\Interfaces\HasAttributesInterface;
 use TrueLayer\Interfaces\RequestOptionsInterface;
 
+/**
+ * Builds a refund request. Refunds are only supported for settled merchant account payments.
+ */
 interface RefundRequestInterface extends HasAttributesInterface
 {
     /**
+     * The payment to refund, either its ID or a retrieved/created payment instance.
+     *
      * @param string|PaymentRetrievedInterface|PaymentCreatedInterface $payment
      *
      * @return RefundRequestInterface
@@ -17,6 +22,8 @@ interface RefundRequestInterface extends HasAttributesInterface
     public function payment($payment): RefundRequestInterface;
 
     /**
+     * A 'cent' value representing the refund amount, e.g. 100 = 1 GBP or 100 = 1 EUR.
+     *
      * @param int $amount
      *
      * @return RefundRequestInterface
@@ -24,6 +31,8 @@ public function payment($payment): RefundRequestInterface;
     public function amountInMinor(int $amount): RefundRequestInterface;
 
     /**
+     * A reference for the refund.
+     *
      * @param string $reference
      *
      * @return RefundRequestInterface
@@ -31,6 +40,8 @@ public function amountInMinor(int $amount): RefundRequestInterface;
     public function reference(string $reference): RefundRequestInterface;
 
     /**
+     * Optional custom key-value data to attach to the refund.
+     *
      * @param array<string, string> $metadata
      *
      * @return RefundRequestInterface
@@ -45,6 +56,8 @@ public function metadata(array $metadata): RefundRequestInterface;
     public function requestOptions(RequestOptionsInterface $requestOptions): RefundRequestInterface;
 
     /**
+     * Send the request to create the refund.
+     *
      * @return RefundCreatedInterface
      */
     public function create(): RefundCreatedInterface;
diff --git a/src/Interfaces/Payment/RefundRetrievedInterface.php b/src/Interfaces/Payment/RefundRetrievedInterface.php
index 08eac3f..0f37c6e 100644
--- a/src/Interfaces/Payment/RefundRetrievedInterface.php
+++ b/src/Interfaces/Payment/RefundRetrievedInterface.php
@@ -6,39 +6,56 @@
 
 use TrueLayer\Interfaces\ArrayableInterface;
 
+/**
+ * Fields and helpers shared by a refund resource regardless of its current status.
+ */
 interface RefundRetrievedInterface extends ArrayableInterface
 {
     /**
+     * The unique ID for the refund.
+     *
      * @return string
      */
     public function getId(): string;
 
     /**
+     * A 'cent' value representing the refund amount, e.g. 100 = 1 GBP or 100 = 1 EUR.
+     *
      * @return int
      */
     public function getAmountInMinor(): int;
 
     /**
+     * The three-letter ISO 4217 currency code the refund was made in.
+     *
      * @return string
      */
     public function getCurrency(): string;
 
     /**
+     * The reference for the refund.
+     *
      * @return string
      */
     public function getReference(): string;
 
     /**
+     * Custom key-value data attached to the refund.
+     *
      * @return array<string, string>
      */
     public function getMetadata(): array;
 
     /**
+     * The date and time the refund was created at.
+     *
      * @return \DateTimeInterface
      */
     public function getCreatedAt(): \DateTimeInterface;
 
     /**
+     * The current status of the refund.
+     *
      * @return string
      */
     public function getStatus(): string;
diff --git a/src/Interfaces/Payment/StartAuthorizationFlowRequestInterface.php b/src/Interfaces/Payment/StartAuthorizationFlowRequestInterface.php
index 25d060b..1f5bef6 100644
--- a/src/Interfaces/Payment/StartAuthorizationFlowRequestInterface.php
+++ b/src/Interfaces/Payment/StartAuthorizationFlowRequestInterface.php
@@ -10,6 +10,10 @@
 use TrueLayer\Interfaces\HasAttributesInterface;
 use TrueLayer\Interfaces\Payment\AuthorizationFlow\AuthorizationFlowResponseInterface;
 
+/**
+ * Builds a request to manually start the authorization flow for a payment, declaring which parts of the
+ * flow your UI is able to support. Cannot be used if `hosted_page` was requested on payment creation.
+ */
 interface StartAuthorizationFlowRequestInterface extends HasAttributesInterface
 {
     /**
@@ -20,26 +24,40 @@ interface StartAuthorizationFlowRequestInterface extends HasAttributesInterface
     public function paymentId(string $paymentId): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Declares that your UI can hand over to the Hosted Payments Page instead of rendering the
+     * authorization flow steps yourself.
+     *
      * @return StartAuthorizationFlowRequestInterface
      */
     public function useHPPCapabilities(): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Declares that your UI can render a provider selection screen. Required for payments with
+     * user-selected provider selection; optional for preselected provider selection.
+     *
      * @return StartAuthorizationFlowRequestInterface
      */
     public function enableProviderSelection(): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Declares that your UI can render a scheme selection screen. Required for payments with
+     * user-selected scheme selection; optional for other scheme selection types.
+     *
      * @return StartAuthorizationFlowRequestInterface
      */
     public function enableSchemeSelection(): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Declares that your UI can render a user account selection screen.
+     *
      * @return StartAuthorizationFlowRequestInterface
      */
     public function enableUserAccountSelection(): StartAuthorizationFlowRequestInterface;
 
     /**
+     * The URI the end user is redirected to after they complete authorization on a third-party site, such
+     * as their bank. Must be registered as an allowed redirect URI in the Console.
+     *
      * @param string $returnUri
      *
      * @return StartAuthorizationFlowRequestInterface
@@ -47,6 +65,11 @@ public function enableUserAccountSelection(): StartAuthorizationFlowRequestInter
     public function returnUri(string $returnUri): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Only applicable if you're regulated and have a direct return URI registered with UK providers,
+     * allowing the authorization flow to redirect directly from the provider to your page without going
+     * via TrueLayer. `returnUri` should still be able to handle a non-direct return, in case the provider
+     * doesn't support this.
+     *
      * @param string $directReturnUri
      *
      * @return StartAuthorizationFlowRequestInterface
@@ -54,6 +77,9 @@ public function returnUri(string $returnUri): StartAuthorizationFlowRequestInter
     public function directReturnUri(string $directReturnUri): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Declares which form input types your UI supports, for providers that require additional inputs (such
+     * as remitter name or account details) during authorization.
+     *
      * @param string[] $types
      *
      * @return StartAuthorizationFlowRequestInterface
@@ -61,6 +87,8 @@ public function directReturnUri(string $directReturnUri): StartAuthorizationFlow
     public function formInputTypes(array $types): StartAuthorizationFlowRequestInterface;
 
     /**
+     * Send the request to start the authorization flow.
+     *
      * @throws SignerException
      * @throws ApiResponseUnsuccessfulException
      * @throws InvalidArgumentException