This recommendation/International Standard specifies an architectural semantics for ODP. This is required to: - provide formalization of the ODP modelling concepts; - assist sound and uniform development of formal descriptions of standards for distributed systems; - act as a bridge between the ODP modelling concepts and the semantic models of the specification languages: LOTOS, SDL, ESTELLE and Z; - provide a basis for uniform and consistent comparison between formal descriptions of the same standard in specification languages that are used to develop an architectural semantics.