The Cache-Control response header is one of HTTP’s more widely known header fields; it allows a site to control how caches handle their data in CDNs, browsers, and elsewhere. This post digs into how it’s used (and misused) on the web. While we’ll see that the vast majority of sites are using it correctly, the data shows that many developers don’t understand some of the details — and that sometimes they give up and send everything that looks like it might help.