The monograph is devoted to the review of studies of terahertz mixers on hot electrons from thin superconducting films NbN. The paper provides an overview of studies of the transformation band of the quasi-optical mixer on hot electrons with two channels of energy relaxation - phonon and diffusion, as well as stability and fluctuation sensitivity of the receiver based on such mixers. The monograph covers a large number of original works (including the author of the monograph) devoted to the study of terahertz mixers on hot electrons based on thin superconducting films. The monograph is intended for senior students, graduate students and beginning researchers working in the field of creating terahertz range devices.