A software testing system uses a graph traversal algorithm to explore a model simulating a software product in order to identify errors in the software product. The model employs a Petri's net construct for maintaining state and governing transitions. In particular, the model mediates between a test driver and the software product. The model-based approach is usable both to validate the design of the software and verify the implementation of that design. Using the Petri net model, the test space is bounded.