Harness is an experimental metacomputing framework based on dynamic reconfigurability and extensible distributed virtual machines. The Harness system seeks to achieve two important goals. First, by enabling reconfiguration of the facilities provided by the virtual machine, Harness is able to provide specialized services appropriate to the platform and adapt to new technological developments. Second, Harness is able to adapt to application needs by configuring the required support services and programming environments on demand. In this paper, we describe the architecture and key features of Harness, and discuss the emulation of two distinct programming environments, namely Javaspaces and PVM, within the Harness environment.