The concept of hybrid modeling is not new, and many researchers have proposed frameworks for combining different methods: not just simulation methods, but many other operations research (OR) modeling approaches. Some of these are described in the following paragraph. Discrete-event simulation (DES) is commonly combined with optimization and most modern DES software tools contain an optimization engine as an add-in. The research philosophy literature distinguishes between methodologies, methods, approaches, paradigms, tools and techniques. This paper focuses more narrowly on models which combine two or more different simulation modeling paradigms: DES, system dynamics (SD) and agent-based modeling (ABM).