In this chapter, a general introduction to wide area networks (WANs) and the relevant popular WAN technologies are given. The definition of WAN, the development of WAN technologies, the international organizations that have important impact on the development of WAN technologies, and the classification of WANs are introduced. The broad-band access technologies for WAN, including the xDSL and Cable Modem, are summarized. ISDN as a typical example of circuit-switched WAN is also briefly explained. In the last section, the WAN transport technologies section, three key technologies, X.25, frame relay, and ATM, are discussed.