This chapter introduces different wireless and mobile communication systems that represent an important technological basis for ubiquitous computing applications. Different communication systems co-exist and vary with respect to many parameters, such as transmission range, data rates, cost, mobility, power consumption, scalability in the number of users, and so forth. The introduction gives a quick chapter overview of prominent communication systems. Next, the necessary minimal knowledge required about wireless transmission and media-sharing technologies is provided. The core of the chapter then provides brief introductions to the classes of wireless networks that are most relevant for ubiquitous computing, in particular wireless wide area networks and wireless distribution, local area, personal area, and sensor networks. A brief description of current convergence efforts follows. Readers should be aware that the wealth of technologies to be considered requires this chapter to remain rather survey-like.