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This paper addresses fundamental limits on the ability to communicate over optical fiber and optical free-space channels. The underlying limits on the sensitivity of optical receivers dictated by quantum effects are reviewed. Receiver sensitivity limitations caused by background light, dark current, post detection amplifier noise, and transmitter imperfection are then considered. Finally the limitations of the channels related to input coupling, loss, and delay distortion are reviewed. Throughout, comparisons are made between the optical systems and their microwave counterparts to highlight differences and the origins of those differences.