Modern frequency standards, such as hydrogen masers, generate very stable frequency references for various applications in communications and metric tracking systems. The frequency stability of some standards currently exceeds 1 X 10-15 for 1000 second averaging times.1 For various reasons of redundancy and cost efficiency there is a need to distribute stable reference signals to remote users without significantly degrading their frequency stability. However, distribution systems generally degrade the frequency stability of transmitted signals by degrading the Signal-to-Noise-Ratio (SNR) of the signal and causing group delay variations in the signal path with respect to time. Thus achieving the required frequency stability in frequency distribution systems has become a difficult technical challenge. The ability to distribute precise frequency references over distances of tens of kilometers will result in considerable cost savings, improved performance and better reliability in the NASA/JPL Deep Space Network (DSN). This ability, for instance, will enable the use of a centralized frequency and timing facility and therefore reduce the number of expensive frequency standards needed in a Deep Space Communications Complex (DSCC). To this end, fiber optic frequency reference distribution system development is an ongoing task at JPL. The present goal is to achieve a transmission stability of 1 X 10-18 for 1000 seconds averaging times over a distance of 29 kilometers. This paper will describe the mechanisms in a distribution system that cause degradation of frequency stability of a transmitted signal. In particular instabilities that result from the use of a semiconductor laser transmitter will be discussed. Finally, it will describe the fiber optic frequency reference distribution systems developed at the Jet Propulsion Laboratory, and now in use at the Goldstone DSCC.