Receiver sensitivity is an important parameter in the design of optical communication systems. Prior results on receiver sensitivity for on-off keying modulated direct detection systems with an optical preamplifier are generalized for the cases of frequency-shift keying and subcarrier modulation. Our results, obtained by using the Gaussian approximation, are compared to those obtained by an 'exact' analysis. The results are also generalized for N-ary modulation schemes and systems having optical amplifiers as linear repeaters. Simple analytic formulae are derived for the maximum system gain of optical amplifiers systems for the following two cases: 1) Constant signal power at each amplifier. 2) Equal amplifier spacing, which ensures constant total power at each amplifier. The functional dependence of system gain on various system parameters, and the effects of optical filtering at the receiver as well as at the intermediate amplifiers, is studied. The analysis is extended to include the effects of random power variations at the output of each amplifier. / Master of Science