This Handbook discusses the technologies covered by ITU-T Recommendations related to Synchronous Digital Hierarchy (SDH), Optical Transport Network (OTN) and Ethernet over Transport (EoT). Each chapter of this Handbook is dedicated to a specific aspect (architecture, multiplexing hierarchy, protection, synchronization, etc.) of SDH, OTN and EoT optical transport networks. Plesiochronous Digital Hierarchy networks, which are at present considered obsolete, and Multiprotocol Label Switching - Transport Profile, which is at a preliminary stage of standardization, are only mentioned in the Preface. Finally, the last chapter of the Handbook discusses automatically switched optical networks (ASON), based on the control planes introduced in addition to transport planes and management planes.