This tutorial paper on coherent optical receivers first describes the principles of heterodyning, homodyning, and phase diversity reception. The limits for receiver sensitivity are stated for various binary modulation formats (ASK, FSK, PSK, DPSK) and their dependence on the linewidth-to-bit-rate ratio is illustrated. The requirements on the local laser oscillator are discussed, including the power necessary to make thermal noise negligible. The principle of the balanced photodetector is explained, and the question of heterodyne efficiency is addressed. Lastly the use of coherent detection for spatial acquisition and tracking is touched upon, and the advantage of coherent reception in the presence of background illumination is pointed out.