The developmentof many complex siumulation applications requires collaborrrtive effortfrom reseachers with different domain knowledge and expertise,possibly at different locations. These simulations systems often require huge computing resources and data set, which may be geo- graphically distribyted, In ordr to support collaborative model development and to cater for the increasing comoplex- ity of such systems, it is necessary to harness distributed resources over the Internet. the emergence of Grid tech nologies provide exciting new opportunities for large scale distributed simulation, enabling collaboration and the use of distributed computing resources, while also facilitating access to geographically distributed data sets. This paper discusses the research challenges that must be addresses before these opportunities can be exploited and presents HLA-GRID_REPast, a system for executing large scale distributed simulations of agent based systems over the Grid.