The WBI Developer Kit provides a convenient and flexible API for programming intermediaries on the web. In simple terms, an intermediary is a programmable proxy. More precisely, intermediaries are computational entities that lie along the HTTP stream and are programmed to tailor, customize, personalize, or otherwise enhance data as they flow along the stream. A caching web proxy is a simple example of an HTTP intermediary. Intermediary-based programming is particularly useful if you want to add functionality to a system but cannot modify the data producer (e.g., server or database) or the data consumer (e.g., browser). WBI is a way to quickly and easily build intermediary applications.