This chapter discussed the impacts on communication integrity of imperfections in the RF link from the output of the transmitter to the input of the receiver. Prior to cellular communications becoming so important, only the LOS communication path was considered and the impact of fading, multiple reflections, and delay spread was regarded as bothersome. Most commonly the solution to the problems introduced by these effects was either to shift to another frequency, to increase the operating power, or install more basestations. With the advent of cellular communications, and digital communications in general, techniques have been developed to overcome the negative impacts of fading without boosting signal levels to the level where they severely impact the operation of other radios. So while many aspects of RF propagation are random, concepts and statistical models have been developed that enable design choices to be made that permit digital communication systems to operate in what would have once been regarded as a hostile environment.