This book covers the various issues related to synchronization and timing in telecommunications networks with emphasis on fundamental principles. A tutorial on the theory of clocks and clock noise analysis and the principles underlying noise transfer in phase- and frequency-locked loops is provided. Timing plays an important role in traditional TDM networks so the notion of clock strata and the role of various applicable standards are explained. The majority of the book addresses the relevance of synchronization in packet networks for providing real-time services. Synchronization in packet networks is a relatively new topic of study this book provides the most current view of the subject, providing material that is not readily available in the open literature. One entire chapter is dedicated to applying synchronization and timing concepts for studying the performance of packet networks for providing services at levels of quality end-users have become accustomed to.