Domain Specific Data Models (DSDMs) have recently been employed with varying degrees of success. This paper explores DSDMs by building to a more formal view of them, what they are, how and when they should be employed, what a DSDM should contain, and approaches on how to leverage DSDMs to build easily integrable components supporting a Model-based Modular Open Systems Approach (MMOSA). Some best practices for employing and leveraging DSDMs are described with the intent of providing directly applicable methods for use in MMOSA systems.