The aim of this book is to analyze and discuss critically the Quality of Service (QoS) of UMTS (Universal Mobile Telecommunication Systems) network. UMTS is a third generation (3G) technology that can be established based on GSM technology. UMTS provides high data rate than GSM technology and supports various applications over internet including VoIP, video call etc. This book reveals not only UMTS technology but also details on Quality of Service(QoS) in UMTS. This work is more concern about the factors that affects the Quality of Service, not focus more on improvement techniques of Quality of Service of UMTS network. Here all the related topics for supporting this work have been presented both theoretically and practically. For practical work OPNET as a simulation tool has been used and it has been applied to different types of UMTS network based on different scenarios.There are various attributes that affects the UMTS network and it is really difficult to find out which one is most effective. That is why several experiments have been done here to find out those attributes and their effects. The details of those experiments have also been discussed here in the last few chapters.