When a receiver is moving in a complicated, multipath environment, it is continuously receiving new contributions with changes in delay, amplitude and polarisation. The problem of exactly analysing the multipath problem is a formidable task, but is usually one without interest. What is important is to get a statistical description of the propagation parameters combined with a knowledge of their relevance, and to get an engineering understanding of the variety of multipath phenomena by reducing the problem as much as possible. In this section, the various propagation parameters are introduced for more detailed treatment in later sections.