A UML diagram that shows the sequence of interactions between two or more entities over time. The entities are represented as parallel vertical lines, with time running from top to bottom. The interaction is shown as messages exchanged between the entities. This is part of the dynamic model of a system.

A sequence diagram is a graphical view of a scenario that shows object interaction in a time-based sequence of what happens first, what happens next. Sequence diagrams establish the roles of objects and help provide essential information to determine class responsibilities and interfaces.