The increasing use of computers for real-time control on board spacecrafts has brought with it a greater emphasis on the development methodology used for such systems. By their nature, spacecraft control computers have to operate unattended for long periods and because of the programmatics of space, systems are subject to a long development cycle. As a result, there are two distinct concerns, the first being that the development approach guarantees functional and timing correctness, the second being that problems, particularly those associated with timing, are considered as early as possible in the spacecraft development life cycle. The European Space Agency has, for a number of years, encouraged the development of software using HOOD. It was thus a natural next step to investigate the incorporation of time within the existing HOOD framework. This has proven to be very beneficial and this book describes the approach developed by the authors for handling Hard Real-Time applications. It describes both the background scheduling theory, provides practical examples of its application to real life problems, and demonstrates how it is used in the various phases of the development of Hard Real-Time systems.