I'm implementing a Web service that returns a JSON-encoded payload. If the service call fails -- say, due to invalid parameters -- a JSON-encoded error is returned. I'm unsure, however, what HTTP status code should be returned in that situation. On one hand, it seems like HTTP status codes are for HTTP: even though an application error is being returned, the HTTP transfer itself was successful, suggesting a 200 OK response. On the other hand, a RESTful approach would seem to suggest that if the caller is attempting to post to a resource, and the JSON parameters of the request are invalid somehow, that a 400 Bad Request is appropriate. I'm using Prototype on the client side, which has a nice mechanism for automatically dispatching to different callbacks based on HTTP status code (onSuccess and onFailure), so I'm tempted to use status codes to indicate service success or failure, but I'd be interested to hear if anyone has opinions or experience with common practice in this matter. Thanks!