The purpose of the present book is not a more or less complete presentation of the theory of Markov chains, which has up to the present time received a wide, though by no means complete, treatment. Its aim is to present only the fundamental results which may be obtained through the use of the matrix method of investigation, and which pertain to chains with a finite number of states and discrete time. Much of what may be found in the work of Fréchet and many other investigators of Markov chains is not contained here; however, there are many problems examined which have not been treated by other investigators, e.g. bicyclic and polycyclic chains, Markov-Bruns chain, correlational and complex chains, statistical applications of Markov chains, and others. Much attention is devoted to the work and ideas of the founder of the theory of chains - the great Russian mathematician A.A. Markov, who has not even now been adequately recognized in the mathematical literature of probability theory. The most essential feature of this book is the development of the matrix method of investigation which, is the fundamental and strongest tool for the treatment of discrete Markov chains.