The reader is briefly introduced to a range of bistable optical resonators including the nonlinear Fabry–Perot interferometer, the cavity ring, the SFR, the double-coupler fiber ring, the fiber double-ring, and a nonlinear optical loop mirror (NOLM)NOLM with feedback. All of these devices can display hysteresis and all can be affected by instabilities. Possible applications are discussed in the physical world.