Passive optical "gyros" as well as active optical "gyros" (i.e. ring laser gyros or RLG) are based on the Sagnac effect. The Sagnac effect is the non-reciprocal phase difference experienced by light propagating along opposite directions in a rotating frame. The active technique for measuring the phase difference consists of placing a gain medium within a ring cavity, i.e. a ring laser. In the presence of rotation, the non-reciprocal phase difference that is generated is automatically transformed to a difference in the laser frequencies along the counterpropagating directions in the ring laser. In the passive optical gyros, the non-reciprocal phase shift generated by rotation must be measured using external techniques. There are two types of passive gyros. One type is based on an interferometer approach in which two light beams propagate along opposite directions around a closed path and the phase shift difference generated by rotation is measured. The sensitivity is enhanced considerably by confining the light propagation within a single mode fiber that is wrapped many times around a cylinder. The other type of passive gyro is based on a ring resonator approach in which an external light source is used to measure the difference in the resonance frequency of the cavity along opposite directions. This resonance frequency difference is proportional to the rotation-induced Sagnac phase difference. Optical cavities may be constructed using bulk-optical, fiber-optical or integrated-optical components. At present there is considerable research in progress that is aimed at the development of passive gyros for a variety of applications from precision navigation to robot control. The performance of such gyros at present are very encouraging. The early emphasis on short term noise is now shifting to dynamic range, scale factor stability, light sources, and integrated optic components. In addition, there has been immense progress in the development of low loss fuzed couplers for both the multiturn interferometer gyro and the resonator gyro. The following references will give a good starting point for the interested reader. The references also include the results of recent work mentioned either individually or included in the listed conference proceedings. Futuristic, novel ideas are discussed in references 16 and 17.