This monograph outlines an approach to building the capabilities and capacity of partner armies for coalition operations through the effective use of Army security cooperation. It is important to clarify two key terms in this study, specifically, the difference between capability and capacity. Simply put, "capability" is the ability to perform a function, and "capacity" is the extent of a capability present. Ongoing operations and emerging missions create competing demands for the Army's capabilities, resulting in requirement gaps that the Army is unable to fill by itself. Although there are other ways to fill capability gaps (e.g., with other Services, contractors, or increased Army end-strength), national and Department of Defense (DoD) strategic guidance emphasizes the need to leverage the capabilities of allies and partners to fill these gaps. Thus, this monograph is concerned with how the Army should focus its security cooperation activities to build the most appropriate capabilities in partner armies. As a supporting entity, it must use its limited security cooperation resources in a way that effectively builds partner army capabilities that support Joint requirements. To do this, the Army cannot work in isolation. Partnering with DoD and other U.S. government agencies provides the solution and also enables the development of partner capacity. This study is part of a larger RAND Arroyo Center effort to assist the U.S. Army in building partner capabilities through enhanced and focused security cooperation. It argues that U.S. Army planners need a comprehensive understanding of the types of capability gaps that partner armies might fill and provides a process for matching them with potential partner capabilities. The study also provides insights into planning associated with Army security cooperation activities and discusses the importance of developing metrics that would allow the Army to assess its security cooperation investment over time.