The Grid allows scientists to design and perform very large-scale experiments that are not possible previously. Many of these experiments produce a large number of data sets and significantly the amount of data being captured, generated, replicated and archived is growing at an astonishing rate. We have designed and prototyped an Active Data System that provides a complete, automated solution to problems that arise in the management and curation of derived data. Significantly, the system allows users to recompute data rather than necessarily store it, and adds a layer that provides efficient access to remotely distributed replicated sources across different middleware stacks.