Communication and collaboration between objects is fundamental to object-orientation. This mirrors the world in which we live, where most human endeavour involves communication, interaction and collaboration between individuals. The author introduces the concept of interaction in more detail. Next are interaction sequence diagrams which for simple interactions provide equivalent models as communication diagrams. An interaction sequence diagram is one of several kinds of UML interaction diagram. Commonly, during requirements analysis or interaction design, object instances are modeled in terms of the roles they play and communicate by message passing. Communication diagrams are the second kind of interaction diagram in the UML notation set. The author examines it in more detail in this chapter. An interaction overview diagram focus on the overview of flow of control in an interaction where the nodes in the diagram are interactions or interaction occurrences. Timing diagrams are another new feature in UML2.0, but were previously included in the book by Douglass on real-time systems development using UML. They are of particular value when specifying how time constraints affect the interaction between lifelines. At last, author examines the issues concerned with interaction diagrams of all types and model consistency.