This International Standard defines the syntax and semantics of the Formal Description Technique LOTOS. LOTOS is in general used for the formal description of distributed, concurrent information processing systems. In particular LOTOS can be used to describe formally the service definitions and protocol specifications of the layers of Open Systems Interconnection (OSI) architecture described in ISO 7498, and related standards, and conformance tests for implementations of OSI protocols and/or OSI functions. It can also be applied for the formal description of other distributed systems, such as telephone switching networks.