The Maritime Theater Missile Defense (MTMD) Forum is an eight-nation informal body formed in 1999 to create international cooperation in the area of Maritime Theater Missile Defense. Although initially set up for maritime ballistic missile defense, the Forum has evolved to consider cooperation in other mission areas, such as land attack and ship self defense. The Forum has established focused working groups to address specific technical areas (e.g., radars, BMC4I) to identify common needs, harmonize future operational requirements and nurture possible collaborative development programs. The Modelling and Simulation Working Group (MSWG) was established in 2001 when the Forum realized that modelling and simulation (M&S) would play an important role in developing, testing and evaluating the projected systems that enable sea based missile defense. The mission of the MSWG is to establish a multinational developed-and-managed M&S framework for Maritime Missile Defense and related mission areas. The purpose of the common M&S framework is to enable development and assessment of MTMD concepts, algorithms, and concepts-of-operations, by providing a providing a setting for national M&S capabilities to be exercised in an international context. The framework will allow nations to analyze performance and interoperability within a coalition capability, and experiment with coalition tactics and doctrine to achieve MTMD objectives. It will support analysis from weapon system to battle force level, permitting nations to identify & analyze capability gaps and relate them to specific systems performance. The MSWG is developing the M&S framework incrementally, following a roadmap that includes collaborative efforts with other MTMD Forum working groups. The primary mechanism for implementing development is a set of international Project Arrangements (PA). Currently, the MSWG is embarking on the implementation of its first PA, among The Netherlands, Germany, and the U.S.