An overview of the important techniques for detection of optical radiation from the ultraviolet, through visible to infrared spectral regions is presented. At the beginning single-point devices are considered. Next, di.erent application circuits used in direct detection systems together with elucidation of the design of front-end circuits and discussion of their performance are presented. Third part of the paper is devoted to advanced techniques including coherent detection. Finally, the updated information devoted to readout of signals from detector arrays and focal plane arrays is included. It is shown that detector focal plane technology has revolutionized many kinds of imaging in the past 25 years.