The basic principles of construction of optical transmission systems, modern definition of models of optical transport networks from the point of view of the International Telecommunication Union (ITU), technologies for multiplexing and interfacing transport networks are presented. Various types of network elements with optical and electrical components and network architectures based on them, including synchronization and control networks, are considered. Attention is paid to the latest directions of development of transport networks based on OTN-OTH, Ethernet, T-MPLS, automatic switched networks (ASONASTN), etc.