This paper reviews the basic optical techniques for controlling microwave signals. First, the theory of how an optical signal can affect the microwave signal will be covered, followed by a review of applications in the various areas to illustrate what has been accomplished in the use of optical techniques to control microwave signals. Optical techniques for processing microwave signals can be broken into three major categories related to how the light affects the microwave signal. First is the photogeneration of charge carriers, in which the light generates holes and electrons. This effect has been used to produce phase shifting, on/off switching, modulation, injection locking, and also direct conversion of DC to microwaves. The second way that light can affect the microwave signal is in a photodlode, by generating a photocurrent to be used for on/off switching, microwave pulse generation, injection locking, or for modulating an output beam for the transmission of a microwave signal. The third technique involves interference effects, or heterodyning, used to generate CW microwave signals. An additional topic covered is the monolithic integration of optical and microwave devices to produce opto- microwave integrated circuits. Finally, some systems applications and requirements for these devices are discussed.