This paper contains a discussion of the Software Development System (SDS), a methodology addressing the problems involved in the development of software for ballistic missile defense systems. These are large real-time, automated systems with a requirement for high reliability. The SDS is a broad approach attacking problems arising in requirements generation, software design, coding, and testing. The approach is highly requirements oriented and has resulted in the formulation of structuring concepts, a requirements statement language, process design language, and support software to be used throughout the development cycle. This methodology represents a significant advance in software technology for the development of software for a class of systems such as BMD. The support software has been implemented and is undergoing evaluation.