The problem book covers all sections of the theory of linear and non-linear circuits of the course of theoretical foundations of electrical engineering. In addition to the tasks, the schemes and parameters of which were chosen by the authors arbitrarily in order to emphasize a particular calculation method, many tasks are presented that arise when studying processes in real electrical devices. Answers are given for all tasks, many tasks have solutions or methodological instructions. When preparing the third edition of the problem book, all the tasks of the second edition were carefully examined. Some tasks that were found to be less interesting and useful have been replaced by new ones. These new tasks, as well as additional tasks placed in all chapters of the problem book, entered the practice of classes with students after the release of the previous edition. The problem book is intended as a teaching aid for students of electric power and electrical engineering specialties of universities.