Achieving the real-time linkage among multiple, geographically-distant, local area networks that support distributed interactive simulation (DIS) is one of the major technical challenges facing the implementation of future large-scale training systems. Data filtering is a technique that can help achieve this real-time linkage. In this paper, we present the results of a detailed study to design and evaluate the performance of data filtering for DIS systems. An approach suitable for the implementation of data filtering in the gateways of DIS networks is given and detailed performance results are presented. The paper is concluded by discussing methods to solve the problem of inaccurate state information at high filtering rates.