Distributed embedded software systems, such as sensor networks and command and control systems, are complex systems with stringent performance, reliability, security, and safety constraints. These are also long-lived systems that must be continually upgraded and evolved to incorporate enhanced functionality. One approach for achieving high quality and evolvability for these systems is to organize them in the form of application-oriented frameworks that allow the system to be composed from orthogonal aspects that can be independently developed, evolved, and certified. In this paper, we define a general framework that allows a distributed embedded system to have relatively independent aspects, including \plug-and-play" capability. We present conditions under which the reliability of the system can be inferred from the reliability of the individual aspects. The approach is illustrated for a framework-based distributed sensor network.