Assurance that a formal specification (system specification or software specification) possesses desired properties can be achieved through (1) manual inspections, (2) formal verification of the desired properties, or (3) simulation and testing of the specification. To achieve the high level of confidence in the correctness required in a safety-critical system, all three approaches must be used in concert. We have developed an specification language, called RSML/sup -e/, and an environment, called NIMBUS, which provides support for all these activities. The three V&V techniques fill complementary roles within the validation and verification process. Manual inspections and visualization provide the specification team, customers, and regulatory representatives the means to informally verify that the behavior described formally matches the desired "real world" behavior of the system. RSML/sup -e/ is a fully formal, synchronous, data-flow language. NIMBUS supports large-scale, distributed simulation of specifications through communications over Microsoft's distributed COM or OMG's CORBA.