This paper discusses a concept of DEVS middleware that would provide a set of services for constructing discrete event models and executing them in simulation or in real-time. Although generic middleware such as CORBA and MPI supports simulation and simulation-oriented middleware such as High Level Architecture Runtime Infrastructure exists, there are many issues that either not addressed at all or have been inadequately addressed in existing systems. DEVS Middleware would improve modeling and simulations due to the beneficial formal properties of DEVS, enhance interoperability among components adhering to the DEVS protocol, and reduce programming complexity by hiding lower level simulation and execution details. The DEVS Middleware concept includes components for model construction and for mapping models onto simulators or real-time execution engines to support the novel concept of "distributed programming by modeling." This paper discusses several sub-layers that have been identified within the overall DEVS middleware concept, existing work that can be brought to bear on developing these layers and the integration needed to create a widely accepted standard.