The book summarizes the main tasks and principles of systematics, characterizes the main systematic categories, provides some rules of zoological nomenclature. A brief outline of the formation of the bird system is given and more common new class systems are described. A systematic review of the class of birds is compiled according to the Wetmore system (1960); it gives brief characteristics of all divisions of the class, including families (morphological features, ecology, distribution, number of vivds).