The concept of a telephone call being routed through a succession of exchanges was first introduced in Chapter 1, extended to the interconnection of several different operators' networks in Chapter 2, and considered from the switching systems' viewpoint in Chapter 6. We are now in a position to examine how the control of such call routeings across the networks (public switched telephone network (PSTN), mobile and others) is achieved. To this end, this chapter first considers the mechanism of signalling between nodes across the networks, and then the principal features of call switching control are introduced. In addition, we consider the broader aspects of control covering multimedia and more complex services.