Abstract : For fiscal year 2014, DoD requested over $11 billion to modify existing weapon systems -- more than 10 percent of its total procurement budget. Traditionally, DoD has acquired proprietary systems, which are costly to upgrade and limit opportunities for competition. Through its Better Buying Power initiatives, DoD has re-emphasized the use of an open systems approach as a way to reduce costs through effective competition. GAO was asked to examine DoD's progress in implementing an open systems approach for UAS acquisitions. This report addresses the following: (1) the characteristics and benefits of an open systems approach; (2) DoD's efforts in implementing an open systems approach for its UAS portfolio; and (3) challenges, if any, DoD is encountering in implementing this approach. GAO analyzed relevant literature and DoD policies on open systems and interviewed agency and private industry officials to understand how open systems have been implemented and their benefits. In addition, GAO assessed acquisition documents and questionnaire responses from 10 current and planned UAS programs to determine their open system strategies. Policies and leadership can help drive DoD's acquisition community to use an open systems approach, but challenges exist. Although DoD and the services have policies that direct programs to use an open systems approach, the Navy is the only service that largely followed the policy when developing its UAS. GAO recommends that the Air Force and Army implement their open systems policies, DoD develop metrics to track open systems implementation, and the services report on these metrics and address any gaps in expertise. DoD partially concurred and stated that its current policies and processes are sufficient. GAO maintains that additional action is needed.