The Department of Defense has a continuing need for satellite communications to satisfy the demand for information exchange for strategic, operations, and tactical warfighters. There is currently a transition planning effort to develop a satellite communications architecture for the 2007-2010 time frame. During this time all three current communication satellite systems; UFO, DSCS, and MILSTAR, are expected to degrade rapidly. As part of the transition planning effort, the U.S. Navy was tasked to form a Mobile Users Study to establish a framework for completing the detailed requirements and engineering work needed to develop the UHF/Mobile User transition plan. Then, as part of the Navy effort, the Naval Postgraduate School's Astronautical Engineering class SE-61 under Professor Brij Agrawal's guidance designed a proposed medium Earth orbit communications satellite. This thesis is a design of the Attitude Dynamics and Control Subsystem for the subject medium Earth MUS communications satellite. The thesis describes and explores the five major steps in designing an Attitude Dynamics and Control Subsystem and focuses on key ADCS related areas that are peculilar to a MEO satellite as compared to a GEO satellite.