The paper summarizes the physics of the photoconductivity process in photoresistors and considers their main characteristics. Methods for calculating electrical circuits are given, many of which have not been previously considered in the technical literature. Diagrams and designs of devices both in operation and for the first time described in the literature are widely presented. The book is designed for a wide range of engineers and technicians involved in the development and use of various devices that use photoresistors, it can be useful to teachers, graduate students and university students.