In this chapter, the operating principles of IFM receivers, some trade-offs in the designs, and several schemes for digitizing the outputs of the video amplitude will be discussed. Since an IFM receiver can measure only one input signal at a time, one continuous-wave (CW) signal will block the receiver from measuring other input signals. This CW problem will be discussed. The overlapping signal problem and some schemes to detect the existence of overlapping signals will also be discussed.