diff --git a/google/cloud/storage/oauth2/anonymous_credentials.h b/google/cloud/storage/oauth2/anonymous_credentials.h
index d127eb0fcfa09..21b54463f5b7b 100644
--- a/google/cloud/storage/oauth2/anonymous_credentials.h
+++ b/google/cloud/storage/oauth2/anonymous_credentials.h
@@ -36,7 +36,10 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-class AnonymousCredentials : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. "
+    "Prefer using the unified credentials "
+    "documented in @ref guac.") AnonymousCredentials : public Credentials {
  public:
   AnonymousCredentials() = default;
 
diff --git a/google/cloud/storage/oauth2/authorized_user_credentials.h b/google/cloud/storage/oauth2/authorized_user_credentials.h
index 240698c47da9c..ca9a04fb57001 100644
--- a/google/cloud/storage/oauth2/authorized_user_credentials.h
+++ b/google/cloud/storage/oauth2/authorized_user_credentials.h
@@ -45,7 +45,10 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-struct AuthorizedUserCredentialsInfo {
+struct GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This struct will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    AuthorizedUserCredentialsInfo {
   std::string client_id;
   std::string client_secret;
   std::string refresh_token;
@@ -60,6 +63,9 @@ struct AuthorizedUserCredentialsInfo {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<RefreshingCredentialsWrapper::TemporaryToken>
 ParseAuthorizedUserRefreshResponse(
     storage::internal::HttpResponse const& response,
@@ -70,6 +76,9 @@ ParseAuthorizedUserRefreshResponse(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<AuthorizedUserCredentialsInfo> ParseAuthorizedUserCredentials(
     std::string const& content, std::string const& source,
     std::string const& default_token_uri =
@@ -106,9 +115,11 @@ class AuthorizedUserCredentials;
 
 /// @copydoc AuthorizedUserCredentials
 template <>
-class AuthorizedUserCredentials<storage::internal::CurlRequestBuilder,
-                                std::chrono::system_clock>
-    : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    AuthorizedUserCredentials<storage::internal::CurlRequestBuilder,
+                              std::chrono::system_clock> : public Credentials {
  public:
   explicit AuthorizedUserCredentials(
       AuthorizedUserCredentialsInfo const& info,
@@ -138,7 +149,10 @@ class AuthorizedUserCredentials<storage::internal::CurlRequestBuilder,
 
 /// @copydoc AuthorizedUserCredentials
 template <typename HttpRequestBuilderType, typename ClockType>
-class AuthorizedUserCredentials : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.") AuthorizedUserCredentials
+    : public Credentials {
  public:
   explicit AuthorizedUserCredentials(AuthorizedUserCredentialsInfo info,
                                      ChannelOptions const& channel_options = {})
diff --git a/google/cloud/storage/oauth2/compute_engine_credentials.h b/google/cloud/storage/oauth2/compute_engine_credentials.h
index 7c4772e438754..603fd331a4b06 100644
--- a/google/cloud/storage/oauth2/compute_engine_credentials.h
+++ b/google/cloud/storage/oauth2/compute_engine_credentials.h
@@ -47,7 +47,9 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-struct ServiceAccountMetadata {
+struct GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This struct will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.") ServiceAccountMetadata {
   std::set<std::string> scopes;
   std::string email;
 };
@@ -57,6 +59,9 @@ struct ServiceAccountMetadata {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<ServiceAccountMetadata> ParseMetadataServerResponse(
     storage::internal::HttpResponse const& response);
 
@@ -67,6 +72,9 @@ StatusOr<ServiceAccountMetadata> ParseMetadataServerResponse(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<RefreshingCredentialsWrapper::TemporaryToken>
 ParseComputeEngineRefreshResponse(
     storage::internal::HttpResponse const& response,
@@ -104,8 +112,11 @@ class ComputeEngineCredentials;
 
 /// @copydoc ComputeEngineCredentials
 template <>
-class ComputeEngineCredentials<storage::internal::CurlRequestBuilder,
-                               std::chrono::system_clock> : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    ComputeEngineCredentials<storage::internal::CurlRequestBuilder,
+                             std::chrono::system_clock> : public Credentials {
  public:
   explicit ComputeEngineCredentials() : ComputeEngineCredentials("default") {}
   explicit ComputeEngineCredentials(std::string service_account_email);
@@ -152,7 +163,10 @@ class ComputeEngineCredentials<storage::internal::CurlRequestBuilder,
 
 /// @copydoc ComputeEngineCredentials
 template <typename HttpRequestBuilderType, typename ClockType>
-class ComputeEngineCredentials : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.") ComputeEngineCredentials
+    : public Credentials {
  public:
   explicit ComputeEngineCredentials() : ComputeEngineCredentials("default") {}
 
diff --git a/google/cloud/storage/oauth2/credential_constants.h b/google/cloud/storage/oauth2/credential_constants.h
index a26831ed4cd81..b9081f9d3afa2 100644
--- a/google/cloud/storage/oauth2/credential_constants.h
+++ b/google/cloud/storage/oauth2/credential_constants.h
@@ -32,14 +32,21 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-// NOLINTNEXTLINE(readability-identifier-naming)
-enum class JwtSigningAlgorithms { RS256 };
+enum class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This enum will be removed in v4.0.0 and later. Prefer using the unified "
+    "credentials documented in @ref guac.") JwtSigningAlgorithms {
+  // NOLINTNEXTLINE(readability-identifier-naming)
+  RS256
+};
 
 /**
  * The max lifetime in seconds of an access token.
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This constant will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 constexpr std::chrono::seconds GoogleOAuthAccessTokenLifetime() {
   return std::chrono::seconds(3600);
 }
@@ -55,6 +62,9 @@ constexpr std::chrono::seconds GoogleOAuthAccessTokenLifetime() {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This constant will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 constexpr std::chrono::seconds GoogleOAuthAccessTokenExpirationSlack() {
   return std::chrono::seconds(300);
 }
@@ -64,6 +74,9 @@ constexpr std::chrono::seconds GoogleOAuthAccessTokenExpirationSlack() {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This constant will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 inline char const* GoogleOAuthRefreshEndpoint() {
   static constexpr char kEndpoint[] = "https://oauth2.googleapis.com/token";
   return kEndpoint;
@@ -74,6 +87,9 @@ inline char const* GoogleOAuthRefreshEndpoint() {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This constant will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 inline char const* GoogleOAuthScopeCloudPlatform() {
   static constexpr char kScope[] =
       "https://www.googleapis.com/auth/cloud-platform";
diff --git a/google/cloud/storage/oauth2/credentials.h b/google/cloud/storage/oauth2/credentials.h
index 962dbc0ce8e3e..1c8570dc6b895 100644
--- a/google/cloud/storage/oauth2/credentials.h
+++ b/google/cloud/storage/oauth2/credentials.h
@@ -44,7 +44,9 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-class Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.") Credentials {
  public:
   virtual ~Credentials() = default;
 
diff --git a/google/cloud/storage/oauth2/google_application_default_credentials_file.h b/google/cloud/storage/oauth2/google_application_default_credentials_file.h
index 0b3f903e01e14..5c9b41d1c053c 100644
--- a/google/cloud/storage/oauth2/google_application_default_credentials_file.h
+++ b/google/cloud/storage/oauth2/google_application_default_credentials_file.h
@@ -32,6 +32,9 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 char const* GoogleAdcEnvVar();
 
 /**
@@ -43,6 +46,9 @@ char const* GoogleAdcEnvVar();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::string GoogleAdcFilePathFromEnvVarOrEmpty();
 
 /**
@@ -54,6 +60,9 @@ std::string GoogleAdcFilePathFromEnvVarOrEmpty();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::string GoogleAdcFilePathFromWellKnownPathOrEmpty();
 
 /**
@@ -64,6 +73,9 @@ std::string GoogleAdcFilePathFromWellKnownPathOrEmpty();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 char const* GoogleGcloudAdcFileEnvVar();
 
 /**
@@ -76,6 +88,9 @@ char const* GoogleGcloudAdcFileEnvVar();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 char const* GoogleAdcHomeEnvVar();
 
 }  // namespace oauth2
diff --git a/google/cloud/storage/oauth2/google_credentials.h b/google/cloud/storage/oauth2/google_credentials.h
index 11c81abdcd717..6c31d7c1de493 100644
--- a/google/cloud/storage/oauth2/google_credentials.h
+++ b/google/cloud/storage/oauth2/google_credentials.h
@@ -44,6 +44,9 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>> GoogleDefaultCredentials(
     ChannelOptions const& options = {});
 
@@ -59,6 +62,9 @@ StatusOr<std::shared_ptr<Credentials>> GoogleDefaultCredentials(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::shared_ptr<Credentials> CreateAnonymousCredentials();
 
 /**
@@ -69,6 +75,9 @@ std::shared_ptr<Credentials> CreateAnonymousCredentials();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateAuthorizedUserCredentialsFromJsonFilePath(std::string const& path);
 
@@ -80,6 +89,9 @@ CreateAuthorizedUserCredentialsFromJsonFilePath(std::string const& path);
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateAuthorizedUserCredentialsFromJsonContents(
     std::string const& contents, ChannelOptions const& options = {});
@@ -105,6 +117,9 @@ CreateAuthorizedUserCredentialsFromJsonContents(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromFilePath(std::string const& path);
 
@@ -132,6 +147,9 @@ CreateServiceAccountCredentialsFromFilePath(std::string const& path);
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromFilePath(
     std::string const& path, absl::optional<std::set<std::string>> scopes,
@@ -146,6 +164,9 @@ CreateServiceAccountCredentialsFromFilePath(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromJsonFilePath(std::string const& path);
 
@@ -170,6 +191,9 @@ CreateServiceAccountCredentialsFromJsonFilePath(std::string const& path);
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromJsonFilePath(
     std::string const& path, absl::optional<std::set<std::string>> scopes,
@@ -184,6 +208,9 @@ CreateServiceAccountCredentialsFromJsonFilePath(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromP12FilePath(std::string const& path);
 
@@ -208,6 +235,9 @@ CreateServiceAccountCredentialsFromP12FilePath(std::string const& path);
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromP12FilePath(
     std::string const& path, absl::optional<std::set<std::string>> scopes,
@@ -231,6 +261,9 @@ CreateServiceAccountCredentialsFromP12FilePath(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromDefaultPaths(
     ChannelOptions const& options = {});
@@ -261,6 +294,9 @@ CreateServiceAccountCredentialsFromDefaultPaths(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromDefaultPaths(
     absl::optional<std::set<std::string>> scopes,
@@ -275,6 +311,9 @@ CreateServiceAccountCredentialsFromDefaultPaths(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromJsonContents(
     std::string const& contents, ChannelOptions const& options = {});
@@ -301,6 +340,9 @@ CreateServiceAccountCredentialsFromJsonContents(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::shared_ptr<Credentials>>
 CreateServiceAccountCredentialsFromJsonContents(
     std::string const& contents, absl::optional<std::set<std::string>> scopes,
@@ -311,6 +353,9 @@ CreateServiceAccountCredentialsFromJsonContents(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::shared_ptr<Credentials> CreateComputeEngineCredentials();
 
 /**
@@ -318,6 +363,9 @@ std::shared_ptr<Credentials> CreateComputeEngineCredentials();
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::shared_ptr<Credentials> CreateComputeEngineCredentials(
     std::string const& service_account_email);
 
diff --git a/google/cloud/storage/oauth2/refreshing_credentials_wrapper.h b/google/cloud/storage/oauth2/refreshing_credentials_wrapper.h
index 85496c491f7c2..2f63398024904 100644
--- a/google/cloud/storage/oauth2/refreshing_credentials_wrapper.h
+++ b/google/cloud/storage/oauth2/refreshing_credentials_wrapper.h
@@ -35,11 +35,16 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-class RefreshingCredentialsWrapper {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    RefreshingCredentialsWrapper {
  public:
   RefreshingCredentialsWrapper();
 
-  struct TemporaryToken {
+  struct GOOGLE_CLOUD_CPP_DEPRECATED(
+      "This struct will be removed in v4.0.0 and later. Prefer using the "
+      "unified credentials documented in @ref guac.") TemporaryToken {
     std::string token;
     std::chrono::system_clock::time_point expiration_time;
   };
diff --git a/google/cloud/storage/oauth2/service_account_credentials.h b/google/cloud/storage/oauth2/service_account_credentials.h
index 7da214edbea8b..cdf1b56f8e5e3 100644
--- a/google/cloud/storage/oauth2/service_account_credentials.h
+++ b/google/cloud/storage/oauth2/service_account_credentials.h
@@ -54,7 +54,10 @@ namespace oauth2 {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-struct ServiceAccountCredentialsInfo {
+struct GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This struct will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    ServiceAccountCredentialsInfo {
   std::string client_email;
   std::string private_key_id;
   std::string private_key;
@@ -70,7 +73,9 @@ struct ServiceAccountCredentialsInfo {
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
-
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<ServiceAccountCredentialsInfo> ParseServiceAccountCredentials(
     std::string const& content, std::string const& source,
     std::string const& default_token_uri = GoogleOAuthRefreshEndpoint());
@@ -86,6 +91,9 @@ StatusOr<ServiceAccountCredentialsInfo> ParseServiceAccountCredentials(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<ServiceAccountCredentialsInfo> ParseServiceAccountP12File(
     std::string const& source,
     std::string const& default_token_uri = GoogleOAuthRefreshEndpoint());
@@ -96,6 +104,9 @@ StatusOr<ServiceAccountCredentialsInfo> ParseServiceAccountP12File(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<RefreshingCredentialsWrapper::TemporaryToken>
 ParseServiceAccountRefreshResponse(
     storage::internal::HttpResponse const& response,
@@ -112,6 +123,9 @@ ParseServiceAccountRefreshResponse(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::pair<std::string, std::string> AssertionComponentsFromInfo(
     ServiceAccountCredentialsInfo const& info,
     std::chrono::system_clock::time_point now);
@@ -123,6 +137,9 @@ std::pair<std::string, std::string> AssertionComponentsFromInfo(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::string MakeJWTAssertion(std::string const& header,
                              std::string const& payload,
                              std::string const& pem_contents);
@@ -136,6 +153,9 @@ std::string MakeJWTAssertion(std::string const& header,
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 std::string CreateServiceAccountRefreshPayload(
     ServiceAccountCredentialsInfo const& info, std::string const& grant_type,
     std::chrono::system_clock::time_point now);
@@ -166,6 +186,9 @@ std::string CreateServiceAccountRefreshPayload(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 StatusOr<std::string> MakeSelfSignedJWT(
     ServiceAccountCredentialsInfo const& info,
     std::chrono::system_clock::time_point tp);
@@ -175,6 +198,9 @@ StatusOr<std::string> MakeSelfSignedJWT(
  *
  * @deprecated Prefer using the unified credentials documented in @ref guac
  */
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 bool ServiceAccountUseOAuth(ServiceAccountCredentialsInfo const& info);
 
 /**
@@ -211,9 +237,11 @@ class ServiceAccountCredentials;
 
 /// @copydoc ServiceAccountCredentials
 template <>
-class ServiceAccountCredentials<storage::internal::CurlRequestBuilder,
-                                std::chrono::system_clock>
-    : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
+    ServiceAccountCredentials<storage::internal::CurlRequestBuilder,
+                              std::chrono::system_clock> : public Credentials {
  public:
   explicit ServiceAccountCredentials(ServiceAccountCredentialsInfo info)
       : ServiceAccountCredentials(std::move(info), {}) {}
@@ -260,7 +288,10 @@ class ServiceAccountCredentials<storage::internal::CurlRequestBuilder,
 
 /// @copydoc ServiceAccountCredentials
 template <typename HttpRequestBuilderType, typename ClockType>
-class ServiceAccountCredentials : public Credentials {
+class GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This class will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.") ServiceAccountCredentials
+    : public Credentials {
  public:
   explicit ServiceAccountCredentials(ServiceAccountCredentialsInfo info)
       : ServiceAccountCredentials(std::move(info), {}) {}
@@ -353,6 +384,9 @@ class ServiceAccountCredentials : public Credentials {
 
 namespace internal {
 
+GOOGLE_CLOUD_CPP_DEPRECATED(
+    "This function will be removed in v4.0.0 and later. Prefer using the "
+    "unified credentials documented in @ref guac.")
 oauth2_internal::ServiceAccountCredentialsInfo MapServiceAccountCredentialsInfo(
     oauth2::ServiceAccountCredentialsInfo info);