The teachings herein generally relate to client-server communications and the delivery of content over computer networks to clients, and provide improved methods, systems, and apparatus for identifying and/or characterizing client devices that are requesting content from a server. For example, based on information sent in a client device's request for content, a web server modified in accordance with the teachings hereof can identify a set of characteristics associated with that client device. Such characteristics might include the model name of the client device, the screen dimensions of the client device, information about the particular operating system or browser name/version it is running, content formats it is capable of consuming, and so on. The web server can use this information to modify and customize its response for the given client device.