Abstract: A method and system in which a programmed digital computer serving as a graphical "engine" is employed to construct an executable model for a complex system using a hierarchy of so called "Colored" Petri nets (CPN's or CP-nets). The operation of the complex system is specified in terms of a graphical net structure and formal net inscriptions, such as arc expressions and guards, which ensure development of a program, or executable model, and associated data structures, properly defining the system. Sophisticated syntax checks and other tools are provided for ensuring that the model so constructed conforms to a set of rules which guarantee valid system behavior.