The medium, that is, a channel between transmitter and receiver, is often heterogeneous, which causes the signal to travel along different paths. Such a channel is known as a multipath channel. In this chapter, we shall study how to characterize a multipath channel given the input and output or just the output. A channel may support a fi nite number of discrete multipaths caused by large refl ectors or boundaries. We will investigate such a discrete channel in Sections 7.1 and 7.2. With a single receiver, we shall receive overlapping echoes of the transmitted signal. In Section 7.1, we shall explore how to estimate the parameters of multipaths, namely delay and strength multipaths from a single sensor output; while in Section 7.2 we show how to use an array sensor to estimate the direction of arrival (DOA) of different multipaths. When there are many scatterers, a large number of closely spaced multipaths with small delay spread are produced. The discrete model is then no longer useful, as it is not possible to estimate so many parameters. In Section 7.3, a different approach is adopted to characterize a scatter channel. Finally, in Section 7.4, a channel, where there are a number of multipaths with large delay spread, is modeled as a fi nite impulse response (FIR) fi lter.