We have so far seen how data is transported from one computer to another over the global Internet, yet an interface is still missing where this basic technology can be used for special services and applications. Tasks that have now become an integral part of life, such as sending electronic mail or using interactive information resources on the World Wide Web, are all founded on a client/server-based interaction model that takes advantage of the possibilities offered by the Internet and its protocols. This chapter focuses on the application layer of the TCP/IP reference model and deals with a variety of services and applications located at this layer. Following a description of the client/server interaction model, directory and name services will be presented before turning to the most important Internet application besides the World Wide Web—electronic mail and the protocols that make it possible. Particular attention will be paid to multimedia applications, such as the popular media streaming, as well as a wide range of other applications.