A generative approach to constructing system specifications is discussed. This process is intended to support design of hierarchical, multicomponent systems. In previous work, a systems design methodology called Knowledge-Based Simulation Design (KBSD) was developed. KBSD focuses on the use of modeling and simulation techniques to build and evaluate models of the system being designed. To represent families of design components, a knowledge representation scheme called system entity structure (SES) is used. Various modeling formalisms may be used for system specifications in the methodology. Thus, an efficient, generative procedure is needed for constructing specifications for systems represented by an SES. It is shown how system specifications can be managed using a canonical SES representation.