Features are disclosed for caching network resources. A system determines whether a resource which does not contain an indication in its associated header that it is cacheable may nevertheless be cached. The determination may be based on observations of a number of requests for the resource over time and from a number of different requesting clients. In response to determining that the resource is cacheable, the system may cache the resource, and in some cases modify the cache control header of resource to facilitate caching. The system may insert fields into the header or otherwise modify the header so that the resource may be cached at a network computing provider, at a requesting client device, or at some other shared or private cache.