This article examines North Korea's aggressive nuclear and missile development efforts and Seoul's response. By applying medium power theory, which is characteristically different from the widely (mis)used concept of "middle power," I explain South Korea's foreign policy behaviors, focusing on its policy toward missile development. South Korea abstained from developing long-range missiles to counter the North Korean missile threat until 2001. I interpret both South Korea's long abstinence and its subsequent efforts to upgrade its missile capability as strategies dictated by South Korea's dual identity: it is a medium power in a global sense and yet a small state within the context of its regional security situation. This aspect is critical to understanding Seoul's security policy and the future of the ROK-U.S. Alliance.