This chapter discusses the continuing importance of military security, noting how International Relations has historically seen security almost entirely in terms of the military dimension. It first examines the impact of the broadening of the concept of security on approaches to the study of its military dimension before considering the key aspects of the traditional approach to military security and some of the most common ways in which states have sought to acquire it historically, such as war, alliances, and nuclear deterrence. The chapter then explores some of the difficulties in acquiring military security and how its pursuit can sometimes reduce, rather than increase, security. In particular, it analyses arms control as a means of achieving military security. Finally, it shows that military security remains an important field to study, even within a significantly broadened understanding of security as a multifaceted concept.