Abstract : This paper examines the current drive to establish a comprehensive Maritime Domain Awareness (MDA) capability for the United States and the role of the Department of Defense (DoD) in this initiative, with a focus on the specific responsibilities for MDA that should be assigned to DoD. The potential for attack of the United States via the Maritime Domain using a commercial vessel carrying a weapon of mass destruction is very real; when coupled with the current shortfall in U.S. MDA capability, the impetus for improving MDA is made apparent. The MDA capability necessary to provide adequate defense against such threats requires an extensive system of sensors and intelligence collection as well as dedicated fusion and analysis to build a common operational picture with which effective decisions regarding maritime threats can be made. The U.S. Coast Guard has taken the lead in establishing U.S. MDA capability. However, DoD has specific responsibilities for Homeland Security and Homeland Defense that mandate a significant role in establishing MDA for the United States. As a result of its global reach, resources, and specific responsibilities, DoD should be required to provide for information and intelligence collection in the forward regions in support of the MDA process, non-deniable vessel surveillance, detection, and tracking in the maritime approaches to the United States, and dedicated analysis and fusion of data to help build the common operational picture that will be the product of the MDA system.