The images, CSS and JavaScript files served up by your Rails application can usually be cached by the web browser, rather than being downloaded (or checked) every time the browser loads a page on your site. For a properly configured site the net effect is faster page load times and a vastly improved user experience. Rails gives you some help here, but it doesn’t happen automatically – you need to configure your web server to set the HTTP Expires header. This article explains how to configure the expires header for a Rails application running behind the nginx HTTP proxy server.