In this chapter we've looked at state machines in Windows Workflow. State machines consist of states, events, and transitions. Windows Workflow provides all the activities we need to model these constituent pieces. State machines will typically be driven by a local communication service or web service requests, and the workflow runtime services, like tracking and persistence, work alongside state machines the same way they work with sequential workflows. Finally, hierarchical state machines enable us to extract common event-driven activities and place them into a parent state.