The aim of this book is to analyse the various behaviour of Retrial queueing system. To motivate, many examples have been quoted.In this book, concepts like priority services,vacation policies,unreliable server,second optional service,negative arrival,vacation interruption and variable service rates have been discussed and 24 models have been developed. System performance measures of these models have been dealt and numerical and graphical studies have been done in elaborate manner. This book will be highly useful to those who are working in the area of queueing theory, Stochastic models in Operation Research and communication Engineering.