This paper presents A-TOS, an aspect-oriented reflective middleware for distributed programming. It provides a very special kind of entities called aspect components that implement global and transversal properties of (distributed) applications like security, fault tolerance, transactions, and so on. Since the application code does not directly refer to the aspect components, A-TOS achieves clean and powerful separation of concerns based on a wrapping composition model. Its adaptability and aspect distribution capabilities make it well suited to aspect-oriented programming in a distributed environment.