This paper explores the application of Base Object Models (BOMs), an emerging XML standard, for rapidly composing simulations and simulation environments. We examine how pattern components, supported by the Interface (IF) BOM and how behavior components, supported by the Encapsulated (ECAP) BOM, can be used to enable simulation composability at design time and dynamically at run-time. We also explore the potential capabilities BOMs and other XML standards provide for the future of web-based simulation and training.