"Equivalent photon approximation. Range of validity, accuracy, etc."

This review is devoted mainly to the two-photon production. We begin with a description of this method. The necessity of such an introduction is, in particular, due to some errors or inaccurate statements when using this method in some papers, and too complicated formulas and methods used in other ones. Apart from the known formulas, a number of new results which are useful for experiment are provided, e. g. a simple and convenient method of estimation of the approximation accuracy. A special appendix is devoted to an analysis of a few errors and inaccurate statements contained in papers of Brodsky, Kinoshita and Terazawa (1970), Greco (1971), Cheng and Wu (1971), Bonneau, Gourdin and Martin (1972), etc