The High Level Architecture (HLA) provides a foundation for technical interoperability among disparate simulations, simulators, live systems, and simulation support tools. Achieving technical interoperability is a significant first step, but it is only part of the interoperability equation. This paper describes other interoperability challenges that federation designers and managers face. These challenges relate to the differences between modeling techniques employed by different simulation federates and the logical interpretation of data shared among federates. Using the Federation Development and Execution Process (FEDEP) as a guide, we identify the key areas where federation developers address simulation interoperability as they apply HLA to their domains particularly in battlespace simulation applications and cite potential sources of support for addressing these issues.