Although first-generation (1G) systems are analogue and second-generation (2G) systems are digital, the underlying principles of the two have much in common in terms of architecture and functionality. In this chapter, the system principles of both generations are explained first and then the main services are described. In fact, the 1G systems have largely been replaced by the 2G. As there are also more of the latter, only a brief description is given of 1G systems. Nonetheless, the discussion provides a good illustration of the factors that have to be taken into account in cellular system design.