A survey is given of the most common types of coherent optical correlators, which are classified as spatial plane correlators, frequency plane correlators and special reference correlators. Only the spatial plane correlators are dealt with rather thoroughly. Basic principles, some special features, advantages and disadvantages mostly are given with references to relevant literature. Optical processing of sideways looking synthetic aperture radar data and the acoustooptical processing of linear FM radar signals are described (rather shortly) as special reference correlaton, of which the first has become the most important application of optical data processing to date. Some advantages and disadvantages of incoherent correlators are given for comparison along with some examples of the most common types. A detailed description of a simple coherent spatial plane correlator is given. Some experimental results are mentioned. The reference function in this correlator is realized as a hard clipped phase plate, which results in a large detection region and a high signal output; only one transforming lens is needed. The experimental results were fully in agreement with the calculated and/or expected values (only non real-time photographic film signal input was investigated). Principles and the use as real-time input devices of ultrasonic light modulators are shortly reviewed. A short description is given of some erasable and reusable film types of which some relevant parameters for real-time operation are listed in a table.