Distributed real-time computer systems replacing many mechanical and hydraulic control systems in high-dependability applications. In these applications a failure in the temporal domain can be as critical as a failure in the value domain. This paper discusses some of the technology trends that explain why distributed embedded real-time systems for high-dependability applications will move into the mainstream. It then investigates the new requirements that must be addressed by the software engineering process. Two of the most important requirements are the design for composability and the systematic validation of high-dependability distributed real-time systems.