diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # ChangeLog for yesod-core
 
+## 1.6.18.5
+
+Document `ErrorResponse` [#1698](https://github.com/yesodweb/yesod/pull/1698)
+
 ## 1.6.18.4
 
 * Fixed a bug where `mkYesod` and other TH functions didn't work for datatypes with explicitly stated type variables, including the case with typeclass constraints. [https://github.com/yesodweb/yesod/pull/1697](#1697)
diff --git a/src/Yesod/Core/Types.hs b/src/Yesod/Core/Types.hs
--- a/src/Yesod/Core/Types.hs
+++ b/src/Yesod/Core/Types.hs
@@ -326,11 +326,28 @@
 -- | Responses to indicate some form of an error occurred.
 data ErrorResponse =
       NotFound
+        -- ^ The requested resource was not found.
+        -- Examples of when this occurs include when an incorrect URL is used, or @yesod-persistent@'s 'get404' doesn't find a value.
+        -- HTTP status: 404.
     | InternalError !Text
+        -- ^ Some sort of unexpected exception.
+        -- If your application uses `throwIO` or `error` to throw an exception, this is the form it would take.
+        -- HTTP status: 500.
     | InvalidArgs ![Text]
+        -- ^ Indicates some sort of invalid or missing argument, like a missing query parameter or malformed JSON body.
+        -- Examples Yesod functions that send this include 'requireCheckJsonBody' and @Yesod.Auth.GoogleEmail2@.
+        -- HTTP status: 400.
     | NotAuthenticated
+        -- ^ Indicates the user is not logged in.
+        -- This is thrown when 'isAuthorized' returns 'AuthenticationRequired'.
+        -- HTTP code: 401.
     | PermissionDenied !Text
+        -- ^ Indicates the user doesn't have permission to access the requested resource.
+        -- This is thrown when 'isAuthorized' returns 'Unauthorized'.
+        -- HTTP code: 403.
     | BadMethod !H.Method
+        -- ^ Indicates the URL would have been valid if used with a different HTTP method (e.g. a GET was used, but only POST is handled.)
+        -- HTTP code: 405.
     deriving (Show, Eq, Generic)
 instance NFData ErrorResponse
 
diff --git a/yesod-core.cabal b/yesod-core.cabal
--- a/yesod-core.cabal
+++ b/yesod-core.cabal
@@ -1,5 +1,5 @@
 name:            yesod-core
-version:         1.6.18.4
+version:         1.6.18.5
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>