The tutorial, written in accordance with the program of the course "Fundamentals of Circuit Theory," discusses the basic concepts and definitions related to circuits and systems, mathematical methods for finding the amplitudes and phases of electrical quantities in linear circuits under sinusoidal and exponential influences. The focus is on the analysis of linear stationary systems and circuits from a single system and information point of view; concepts about system operators, special functions are introduced; chains under exponential action are considered in more detail. The book is intended for students of radio engineering faculties of all specialties and forms of education. It can also be used by graduate students and teachers in studying the basics of circuit theory.