A method for developing a family of complex systems having a common software architecture platform. The method entails forming a functional requirements specification (FRS) which includes use cases that describes interaction of users with said complex systems in terms of abstract concept and forming a requirements object model which explains the abstract concepts in terms of a structured vocabulary. The use cases are developed simultaneously with the requirements object model. The FRS specifies the interaction of the complex systems with their users, i.e., the FRS specifies the activities which are performed in the interaction of the complex systems with their users, i.e., `what` the complex systems do. This is preferably done in terms of the use cases which are usually written in a natural language such as English. The requirements object model specifies the concepts to which the activities specified in the FRS relate.