A sample of radar data is composed of either interference alone or interference plus target echoes. The interference is, at a minimum, receiver noise and might also include clutter echoes, electromagnetic interference (EMI) from other transmitting sources (e.g., radars, television stations, cellular telephones), and hostile jamming. Most of these interfering signals are noise-like and are therefore modeled as random processes. Occasionally, a target echo will also be present in a particular sample of radar data. One of the major tasks of a radar system is to detect the presence of these targets when they occur. As was seen in Chapter 3, this is generally accomplished by threshold detection. In this chapter, common models for the statistics of target echoes are discussed, with an emphasis on the traditional Swerling models. The effect of these models on radar detection performance is considered in Chapter 15.