This International Standard defines the Mission Operations (MO) Message Abstraction Layer (MAL) in conformance with the service framework specified in Mission Operations Services Concept, CCSDS 520.0- G-3, Mission Operations Services Concept. The MO MAL is a framework that provides generic service patterns to the Mission Operation services defined in Mission Operations Services Concept, CCSDS 520.0-G-3. These Mission Operations services are defined in terms of the MAL. This International Standard defines, in an abstract manner, the MAL in terms of: a) the concepts that it builds upon; b) the basic types it provides; c) the message headers required by the layer; d) the relationship between, and the valid sequence of, the messages and resulting behaviours. It does not specify: a) individual implementations or products; b) the implementation of entities or interfaces within real systems; c) the methods or technologies required for communications. The scope and field of application are furthermore detailed in subclause 1.3 of the enclosed CCSDS publication.