The paper examines the technology of composability in simulation systems. Composability refers to the ability of a simulation to be flexibly configured to adapt to a range of missions, scenarios, simulation models, hardware environments, and security configurations. Composability confers maximum flexibility to the usage of the simulation. Simulation composability is a requirement of the Joint Simulation System (JSIMS). JSIMS is currently being developed by the US Department of Defense and is intended To deliver commander and command staff training. The paper examines simulation composability from the JSIMS perspective and explores the overall technical approach and the related issues.