Operators of today's high-speed optical communications networks are required to deliver error-free transmission and continuously increasing bit rates. The complexity of synchronous optical networks (SONET, SDH) and high-speed data communications systems (Gigabit Ethernet) allows little tolerance for signal impairment. As a result, strict requirements on performance metrics such as synchronization, jitter, bit error rate (BER), and availability are all necessary. This paper reviews jitter and how it may affect the performance of optical communications systems. This will include discussion of important jitter parameters, sources of jitter, and test methods used to measure jitter in optical networks. Measurements of various jitter scenarios on OC-192 transmission will be presented, and the impact of optical signal impairments on OC-192 performance will be shown. Finally, the relationships between optical signal impairments, observed jitter, and 10GbE stressed-eye tests will be discussed.