Abstract: In a Web-Based federated simulation system, a group of simulation models residing on different machines attached to the Internet, called federates, collaborate with each other to accomplish a common task of simulating a complex real world system. To reduce the cost of developing and maintaining simulation models and facilitate the process of building complex collaborative simulation systems, reuse of existing simulation models and interoperability between disparate simulation models are of paramount importance. Moreover, to make such a system highly extensible, the individual federates, which could reside on the same host or physically distributed hosts, should be able to freely join and leave a federation without full knowledge of its peer federates. Simply put, an ideal simulation system should allow for quick and cheap assembly of a complex simulation out of independently developed simulations and at the same time allow the participating simulations to have maximum independence. Fortunately, this is made possible by some emerging technologies, notably Jini and the eXtensible Markup Language (XML). In this paper, we will introduce Jini and XML and present the design and prototype implementation of a Web-based federated simulation system using Jini and XML.