In designing a system, the systems engineer must thoroughly understand the requirements, constraints, and values established by the customer, must plan and schedule a program to satisfy these conditions, while also considering cost, reliability, and maintainability. Using an APU system as an example, the author traces the development of a design for a control system.First the control system is evaluated as to requirements, constraints, reliability, cost, and maintenance. A preliminary design is developed after conducting various studies, and a detail design implemented. Data obtained from system simulation aid in the development pattern of the control system. The development of the component and subsystem is also initiated, with the system development an extension of this work. The staffing and management of such a program are discussed.