Three distinct techniques exist for distributing an ultrastable frequency reference over optical fibers. For the distribution of a microwave frequency reference, an amplitude-modulated continuous wave (cw) laser can be used. Over kilometer-scale lengths this approach provides an instability at 1 s of similar to 3x10(-14) without stabilization of the fiber-induced noise and similar to 1x10(-14) with active noise cancellation. An optical frequency reference can be transferred by directly transmitting a stabilized cw laser over fiber and then disseminated to other optical and microwave regions using an optical frequency comb. This provides an instability at 1 s of 2x10(-14) without active noise cancellation and 3x10(-15) with active noise cancellation [Recent results reduce the instability at 1 s to 6x10(-18).] Finally, microwave and optical frequency references can be simultaneously transmitted using an optical frequency comb, and we expect the optical transfer to be similar in performance to the cw optical frequency transfer. The instability at 1 s for transfer of a microwave frequency reference with the comb is similar to 3x10(-14) without active noise cancellation and < 7x10(-15) with active stabilization. The comb can also distribute a microwave frequency reference with root-mean-square timing jitter below 16 fs integrated over the Nyquist bandwidth of the pulse train (similar to 50 MHz) when high-bandwidth active noise cancellation is employed, which is important for remote synchronization applications.(c) 2007 American Institute of Physics.