This chapter describes four techniques for client-side JavaScript networking. The first, XMLHttpRequest, is well-known and widely-used in the “Ajax” application architecture. This API is by far the most important of the four, and the bulk of the chapter is devoted to it. The chapter also demonstrates the JSONP technique for Ajax-style networking with the <script> tag, as well as “server push” or “Comet” style networking with the new EventSource API, and bidirectional socket-style networking with WebSockets.