Studying complex systems theory means studying the systems approach and systems analysis. The book analyzes the concept of complexity and system. The signs of a complex system are considered. systematics of complex systems is given. The difference between system approach and system analysis is shown. Information units as elements of complex systems are described. Types of complexity are described. The book describes the systematics of complex systems. System principles, on the basis of which a complex system is formed, are described. a dichotomous analysis of simple and complex systems is given. the content of a number of complex systems is revealed: organizational, technical, technological, ergatic, organizational and technical, data systems, The autopoiesis of a complex system is described. a brief description of cyber-physical systems is given. The system of the Internet of Things as a complex system is described. The category of conditionally complex systems that are complex under some conditions and simple under others is described. A transportation network and routing procedures in it are described. Living systems and the divisibility of living systems are described.