Either Last-Modified or Etag is sufficient. Most servers will add both, though. Edit: The difference is that Last-Modified allows clients to use a heuristic to

Either Last-Modified or Etag is sufficient. Most servers will add both, though. Edit: The difference is that Last-Modified allows clients to use a heuristic to determine if the response should be cached for a certain duration (unless explicit Cache-Control or Expires headers are used). The heuristic isn't specified, but a 10% fraction of Date - Last-Modified is suggested, and in fact, that is what e.g. Internet Explorer does.