diff --git a/repository/Seaside-Core.package/WAAbstractFileLibrary.class/instance/entityTagFor..st b/repository/Seaside-Core.package/WAAbstractFileLibrary.class/instance/entityTagFor..st
new file mode 100644
index 000000000..6e6237034
--- /dev/null
+++ b/repository/Seaside-Core.package/WAAbstractFileLibrary.class/instance/entityTagFor..st
@@ -0,0 +1,11 @@
+private
+entityTagFor: aStringOrByteArray
+	| hash base64 |
+	hash := GRPlatform current secureHashFor: aStringOrByteArray.
+	"etags have to be delimited by double quotes"
+	base64 := GRPlatform current base64Encode: hash asByteArray.
+	^ String new: base64 size + 2 streamContents:  [ :stream |
+		stream
+			nextPut: $";
+			nextPutAll: base64;
+			nextPut: $"]
\ No newline at end of file
diff --git a/repository/Seaside-Core.package/WAFileLibrary.class/instance/handle..st b/repository/Seaside-Core.package/WAFileLibrary.class/instance/handle..st
index 5871d99fc..d64b746fc 100644
--- a/repository/Seaside-Core.package/WAFileLibrary.class/instance/handle..st
+++ b/repository/Seaside-Core.package/WAFileLibrary.class/instance/handle..st
@@ -16,7 +16,14 @@ handle: aRequestContext
 					contentType: (self mimetypeOf: selector) ] ]
 		ifFalse: [
 			aRequestContext respond: [ :response |
-				response
-					cacheFor: self cacheDuration;
-					document: (self documentOf: selector)
-					mimeType: (self mimetypeOf: selector) ] ]
\ No newline at end of file
+				| document entityTag |
+				document := self documentOf: selector.
+				entityTag := self entityTagFor: document.
+				aRequestContext request ifNoneMatch = entityTag
+					ifTrue: [ response notModified ]
+					ifFalse: [ 
+						response
+							cacheFor: self cacheDuration;
+							entityTag: entityTag;
+							document: document
+							mimeType: (self mimetypeOf: selector) ] ] ]
\ No newline at end of file
diff --git a/repository/Seaside-Core.package/WAFileMetadataLibrary.class/instance/handle..st b/repository/Seaside-Core.package/WAFileMetadataLibrary.class/instance/handle..st
index 71ee672b6..cbf883920 100644
--- a/repository/Seaside-Core.package/WAFileMetadataLibrary.class/instance/handle..st
+++ b/repository/Seaside-Core.package/WAFileMetadataLibrary.class/instance/handle..st
@@ -22,7 +22,14 @@ handle: aRequestContext
 					contentType: fileResource mimeType ] ]
 		ifFalse: [
 			aRequestContext respond: [ :response |
-				response
-					cacheFor: fileResource cacheDuration;
-					document: fileResource contents value
-					mimeType: fileResource mimeType ] ]
\ No newline at end of file
+				| document entityTag |
+				document := fileResource contents value.
+				entityTag := self entityTagFor: document.
+				aRequestContext request ifNoneMatch = entityTag
+					ifTrue: [ response notModified ]
+					ifFalse: [ 
+						response
+							cacheFor: fileResource cacheDuration;
+							entityTag: (self entityTagFor: document);
+							document: document
+							mimeType: fileResource mimeType ] ] ]
\ No newline at end of file
diff --git a/repository/Seaside-Core.package/WARequest.class/instance/ifNoneMatch.st b/repository/Seaside-Core.package/WARequest.class/instance/ifNoneMatch.st
new file mode 100644
index 000000000..9ae4e77ec
--- /dev/null
+++ b/repository/Seaside-Core.package/WARequest.class/instance/ifNoneMatch.st
@@ -0,0 +1,5 @@
+accessing-headers
+ifNoneMatch
+	"The If-None-Match HTTP request header makes the request conditional."
+
+	^ self headerAt: 'if-none-match' ifAbsent: [ nil ]
\ No newline at end of file
diff --git a/repository/Seaside-Core.package/WAResponse.class/instance/entityTag..st b/repository/Seaside-Core.package/WAResponse.class/instance/entityTag..st
new file mode 100644
index 000000000..40408d3db
--- /dev/null
+++ b/repository/Seaside-Core.package/WAResponse.class/instance/entityTag..st
@@ -0,0 +1,5 @@
+convenience-caching
+entityTag: aString
+	"The ETag (or entity tag) HTTP response header is an identifier for a specific version of a resource."
+
+	self headerAt: 'ETag' put: aString
\ No newline at end of file
diff --git a/repository/Seaside-Core.package/WAResponse.class/instance/notModified.st b/repository/Seaside-Core.package/WAResponse.class/instance/notModified.st
new file mode 100644
index 000000000..c8193bb14
--- /dev/null
+++ b/repository/Seaside-Core.package/WAResponse.class/instance/notModified.st
@@ -0,0 +1,5 @@
+status
+notModified
+	"ndicate that there is no need to retransmit the requested resources. It is an implicit redirection to a cached resource."
+	
+	self status: WAResponse statusNotModified
\ No newline at end of file