The Unified Modeling Language (UML) from the Object Management Group is the de facto standard for object modeling. This chapter explains how selected UML models can be used in commercial model-based testing (MBT) products. Practitioners divide into two distinct schools of thought on using UML for software development: the top-down school and the bottom-up school. The chapter focuses on the following requirements level, behavioral models: use case diagram, activity diagram, state machine diagram and sequence diagram. Because activity diagrams are "object-oriented flowcharts", they inevitably have all the limitations of ordinary flowcharts. Statecharts, and their special, simplified case, finite state machines, are the control model central to UML 2.5. The UML sequence diagram is unique in the world of mainline models: combines both the IS and the DOES views of a single use case. The UML Activity Diagrams are used by several commercial MBT products for test case generation.