In this chapter we have provided an overview of mobility in the WAN environment and issues which result from mobility. For the most part, these issues are due to inherent conflicts between conventional WAN technology and mobile systems. Mobility directly challenges many of the underlying assumptions of the conventional WAN world, most notably the relative stationarity of host location. Historically, mobile systems arose from the connection-oriented paradigm of telephony. The following chapter describes cellular systems, the archetype of the mobile world.