An activity diagram is a type of UML diagram that represents workflows of stepwise activities and actions with support for choice, iteration, and concurrency. It is essentially an advanced version of a flowchart that models the flow from one activity to another activity. Activity diagrams are intended to model both computational and organizational processes, as well as the data flows intersecting with the related activities. They can be used to describe dynamic aspects of a system and how activities are coordinated to provide a service at different levels of abstraction.

Activity diagrams are constructed from a limited number of shapes, connected with arrows. The most important shape types include stadia, which represent actions; diamonds, which represent decisions; bars, which represent the start (split) or end (join) of concurrent activities; a black circle, which represents the start (initial node) of the workflow; and an encircled black circle, which represents the end (final node). Arrows run from the start towards the end and represent the order in which activities happen.

Activity diagrams can be used to demonstrate the logic of an algorithm, describe the steps performed in a UML use case, illustrate a business process or workflow between users and the system, simplify and improve any process by clarifying complicated use cases, and model software architecture elements such as method, function, and operation.

There is no privileged order for creating diagrams, but activity diagrams are typically created earlier than sequence diagrams. Activity diagrams represent workflows and processes and show sequence and concurrency of steps, and do so without showing design details of modules, classes, and methods.