This chapter, Climate Change as a National Security Issue, looks at what various scholars and practitioners have said about the intersection between climate change and national security. The intent of this chapter is to provide a sample overview of the various experts from academia, government, and industry in order to characterize the extensive breadth and depth of the focus on climate security. First, a brief history of the linkage is canvased within the academic literature. Second, a history of the policy discussions and decisions, primarily from a U.S. perspective, is provided. Third, a history of the concept of climate security is discussed within U.S. and international perspectives. Fourth, a short summary is presented of think tanks and their input to the climate security field, using U.S. organizations. Last, the chapter wraps up with a listing and brief summary of the 18 agencies that make up the U.S. intelligence community, what their primary role is, and some thoughts on how they treat climate-related information.