This book is an introduction to queueing theory. It provides a thorough treatment of Markov processes, renewal theory, regenerative processes, and random walks and discusses in some detail basic models including G1/G/I queue, risk processes, and dams. Particular attention has been given to modern probablistic points of view, as alternatives to traditional methods. Exercises are included and a survey of mathematical prerequisites are given in an appendix. This new edition has been thoroughly updated.