A Communication Diagram models the structural dimension of a scenario by graphically illustrating the interactions between actors and instances. Communication Diagrams provide alternative representations of a Sequence Diagram, each one modeling a different execution thread (or cscenario) through a use case by showing the flow or sequence of messages between associated objects. It is possible to have multiple Communication Diagrams for a single use case.

Communication Diagrams provide help with the architectural design of the system by providing a visual representation of the structure of object communications. They can be constructed very quickly, enabling experiments to be carried out with alternative designs based on different objects as collaborators and controllers, and with different messages passing between them.

The first step in developing a Communication Diagram is to add actors and instances. The second step is to link items with message flows, which become the mechanism for carrying events or operations. Instances must be stimulated by either events or operations carried by message flows.