The order and presentation of Foundations of Queueing Theory is drawn from Professor Prabhu's extensive experience as a researcher, teacher, expositor, and editor. The book deals with the foundations of queueing theory and is intended as an advanced text for courses on queueing theory, and as a professional reference to queueing research. The results of queueing theory are established within the context of the book's instructional framework. The central results of queueing theory are stated in the form of theorems for accessibility and reading ease. Current research has shown the need to pay more attention to the basic concepts and techniques of queueing theory. These include the busy period, imbedded chains, regeneration points, Wiener-Hopf technique, time-reversibility, output, vector Markov processes, remaining workload and completion times. The use of these concepts and techniques considerably simplifies the analysis of models involving last come, first served queue discipline, priorities, networks, set-up times and server vacations. It is these conceptional techniques that form the foundations of queueing theory.