During the past two decades, a number of methods have been proposed for performing bipolar spatial filtering operations with incoherent optical systems. These methods are conveniently categorized as to being based on geometrical optics or on diffraction optics. Some methods require a mechanical scanning of the input object, others are fully parallel in their operation. Still others are optically parallel, but employ an image plane scan to provide an electrical signal representation of the bipolar processed image. This paper reviews these methods and, to a limited extent, compares them.