This book provides a clear, understandable, and motivated account on the subject that spans both conventional and modern materials about discrete event systems, material that, up to now, has been presented in the literature in different fields, such as the graph theory, the probability theory, the automata's theory, and the queueing theory. The book gives a complete introduction to the discrete-event system theory and simultaneously applies the theory to practical problems. The book gives students of computer sciences, system sciences, and of electrical engineering, a clear, unambiguous, and relevant account of discrete-event systems. Numerous illustrations are included for better understanding. Problems as well as their solutions are included in each chapter. It can be used as a basic introduction for undergraduates and graduate students. Although it is logically self-contained, it presupposes the mathematical maturity acquired by students with two years of calculus.