Conventional heterodyne detection is useful in a number of configurations, including the detection of scattered or reflected radiation from a moving target (Doppler radar), communications, spectroscopy, and radiometry. Its use has been demonstrated in many regions of the electromagnetic spectrum including the radiowave, microwave, infrared, and optical. Its advantages as a detection technique are well known: high sensitivity, frequency selectivity, and strong directivity. For radar applications, it provides a major method of recovering desired signals and removing clutter. The significant improvement in sensitivity that it provides over direct detection arises from knowledge of the Doppler frequency (also called the heterodyne frequency or the intermediate frequency (IF)) which permits a narrow receiver bandwidth centered about the IF. In such applications, obtaining a reasonably high signal-to-noise ratio (SNR) requires 1) a good knowledge of the velocity of the source or target, 2) a stable yet tunable local oscillator, 3) a target or source which presents a minimum of frequency broadening and 4) at least several photons per measurement interval. These conditions are frequently not adhered to by actual systems, particularly in the infrared and optical, giving rise to detection capabilities which are well below optimum. In this chapter, we study the performance and requirements of a number of alternative heterodyne receiver configurations. In particular, we consider two basic systems which are intrinsically nonlinear, the first by virtue of the multiple-quantum detection process itself, and the second by virtue of the mixing configuration and the electronics following the detector.