In this Thesis, the generation of a chaotic carrier by semiconductor lasers is theoretically studied. Three different encoding techniques (Chaotic Modulation, Chaotic Shift Keying, and Chaotic Masking) are employed and the performance of an optical chaotic communications system for different receiver configurations is evaluated. It is proved that chaotic carriers allow the successful encoding and decoding of messages with up to 10 Gb/s bit rate.Focusing on the Chaotic Modulation encoding method, the performance of a back-toback chaotic communications system is studied when two different decoding methods are followed. In both closed-and open-loop configurations, the normalized decoding method that is widely used in literature provides significantly different results in the estimation of the system's performance compared to the more realistic method of photocurrent subtraction, proposed in this study.The performance of an open-loop configuration is further studied for Chaotic Modulation and Chaotic Shift Keying encoding techniques when various parameter values of the transmitter laser deviate from the corresponding receiver laser values. In this so-called parameter mismatch analysis, it is indicated that the system's performance is highly dependant on the level of parameter values deviation, as well as the coupling strength between transmitter and receiver.Finally, the effects of transmission in optical fiber are studied for a chaotic communications system when CM and CSK encoding techniques are employed for various message bit-rates. The downgrading appearing to the system's performance due to fiber transmission impairments is confronted by the employment of various dispersion management techniques. The performance of such a system is further evaluated when two different receiver configurations (open-and closed loop) are utilized under weak or strong injection conditions, when either anti-reflection coated lasers or high-reflection coated lasers are employed in the system's receiving end.