Optics enable the realization of significant delays without losses or resonances with very high quality factors (Q). Optical resonators with Q factors on the order of a billion can be employed to stabilize microwave sources and generate signals with extremely low phase noise in the centimeter or millimeter wave range. Optoelectronic oscillators are even more efficient when the reference optical resonator is active or replaced by a mode-locked laser. Of course, in this case, the noise contributions associated with the use of optics must be studied in detail. The dynamics of these systems are also highly complex, requiring advanced nonlinear models that include noise sources. Under certain conditions, the optical resonator itself can operate in a nonlinear regime, delivering a frequency comb that, when detected by a photodiode, provides a stable microwave signal.