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This chapter derives signal-to-noise ratio (SNR) of the continuous-wave (CW) Doppler radar system for physiological monitoring. This derivation assesses the theoretical limits of the radar system and determines the factors that affect the limits so that design decisions can be made appropriately. The radar equation is used to estimate the received power, taking into account the range to the target, the transmitted power, the radar cross section (RCS), the antenna gain, the wavelength, and the range. Noise sources include radio frequency (RF) phase noise from the oscillator, environmental thermal noise, and baseband 1/f noise of the mixer and of the baseband signal-conditioning circuits. The SNR depends on the amount of noise at the mixer output from each of these sources. The chapter analyzes the variation of SNR with range, RCS, and the amount of physiological motion. It describes the effects of near-field operation on the antenna gain.