The growing popularity and common usage of real-time reactive systems on various domains force activities that lead to the improvement of the development stage by applying several software engineering methods e.g.: MDA (Model Driven Architecture) or executable modelling, which could provide a working prototype of a system in the early stage of development. It is also required to have some tools that enable the designing, testing, simulation and running of the software preserving time constraints and interacting with its environment. RAT (Reactive Appliance Toolkit) is a kind of software that allows us to create of an executable model of a real-time reactive system in UML and Real-Time Java and, simulates its behavior and verifies time constraints.