This chapter discusses the basic principles underlying optical processors. Optical image processing refers to manipulation of image by optical means. Just as time-dependent electrical signals can be decomposed into Fourier frequency components, images can be decomposed into spatial frequency components, for instance, by using an optical processor. The attractiveness of optical image processing stems from the speed with which the signals are processed. Fourier tranforms are computed in the time it takes for the light to propagate from the object to the back focal plane of the lens. The generic optical processor configuration forms the basis for understanding modern optical processors such as the schlieren system. The system consists of two convex lenses that image a point light source from the focal plane of the first lens onto the back focal plane of the second one, referred to as the optical Fourier transform plane.