501 Not Implemented means the server does not support the functionality required to fulfill the request. It is the right response when the server does not recognise the request method and cannot support it for any resource. If the method is known but not allowed for this particular resource, 405 Method Not Allowed is correct instead.
Servers also answer 501 when a request uses a Transfer-Encoding they do not understand. A 501 is heuristically cacheable.
Example response
HTTP/1.1 501 Not Implemented
Date: Fri, 06 Nov 2026 10:15:32 GMT
Content-Type: text/plain
Content-Length: 50
Method PROPFIND is not implemented by this server.SEO and redirect checking
Crawlers fetch pages with GET, which every general-purpose server must support (as well as HEAD), so 501 should never reach them. If a check with the method set to HEAD returns 501 while GET works, the server does not implement HEAD, which confuses link checkers and monitoring tools. Search engines treat a persistent 501 like other 5xx errors.
Common causes and use cases
- A client used a method that the server does not know, such as
PROPFINDon a server without WebDAV, or a custom method. - The request uses a
Transfer-Encodingthat the server does not support. - A minimal or embedded HTTP server implements only
GET.