Formal methods based on state-space enumeration, such as Timed Automata and Time Petri Nets (TPN), have been proposed for designing and validating reactive real-time systems. The great expressiveness of these methods is counterbalanced by the increased complexity of the analysis, which may grow exponentially. Furthermore, the enumerated state-space needs to be inspected to identify critical behaviors with respect to sequencing and timing requirements. This naturally leads to the implementation of tools supporting the different stages of the development process. In this paper we present Oris, an environment for building, simulating, analyzing and validating complex real time systems specified in terms of an extended TPN formalism, named Preemptive Time Petri Nets. Oris includes not only the state-space enumeration engine, but also a number of modules which ease user interaction, and make it usable also by a designer with no specific experience in formal modelling.