Current technologies for distributed application development, while being well-adapted for the client-server model, do not adequately address emerging application domains, such as multimedia, embedded or mobile applications. The black box approach, used to achieve distribution and OS transparency, prevents application developers from adapting execution environments to the semantics and constraints of their application domain, thus leading to a growing number of ad-hoc solutions which are closed, static and poorly interoperable. As a solution, we present a reflexive and dynamically adaptable execution environment,/sup 1/ based on reification of hardware resources and a standalone dynamic code generator. This architecture allows dynamic construction of specialized OS or middleware components.