Documents include Paper and Presentation. Simulation Interoperability Standards Organization (SISO) SIW Conference Paper XML has become a popular representation format for data, both in modeling and simulation and elsewhere. However, XML's design choice of a text-based format also makes XML data files much larger than binary files, making XML languages difficult to use in bandwidth-constrained military applications. This limitation has resulted in several ad-hoc attempts to make XML more compact, each of which tends to be incompatible with the other. Efficient XML Interchange (EXI) is a World Wide Web Consortium (W3C) Working Draft for the compact and efficient representation of the XML infoset. EXI is designed to be generally applicable to all XML documents, and lays the foundation for a unified format for compact XML document representation. This paper presents compactness results for several popular modeling and simulation XML file formats, including Distributed Interactive Simulation (DIS), Scalable Vector Graphics (SVG) and Extensible 3D Graphics (X3D). Recent commercial and open source EXI implementations are also described. Naval Postgraduate School, Monterey, CA.