Over the last several years there has been a proliferation in the use of Computer Based Training (CBT) systems. One of the key factors in the growth of CBT systems was the advent of authoring tools that allowed the courseware developer to focus on the content vice the supporting infrastructure. The production of a core infrastructure that abstracts out many of the underlying details is on the key goals of the Joint Simulation System (JSIMS) program. As the model developers develop the content, they will then be integrated with the common JSIMS core to provide a M&S capability with reduced developmental costs. This paper presents an overview of the JSIMS architecture focusing on the mechanisms for achieving composability, scalability, distributability, and increased training efficiency. The JSIMS architecture contains four layers, each addressing more abstract levels of functionality. The lowest layer represents the virtual network. The JSIMS Object Services layer adds an HLA-compliant RTI as well as an Object Management Framework that allows end-to-end object management. A support services layer adapts each application (resident in the application layer) to the underlying infrastructure. The construction of the layered architecture allows the developer to focus on the development content, in this case the applications and mission space objects (MSOs), which is the true value added portion of a model.