This chapter discusses the concept of environmental security. It explains the way environment and climate change have both broadened and deepened the issue of security. It describes the evolution of the concept as a merger of international environmental agreements, efforts to contest the meaning and practice of security, the proliferation of new security issues in the post-Cold War era, recognition that environmental and climate changes pose grave risks to human well-being, and the growing community of research practice that seeks to build peace through natural resource management. The chapter examines the different meanings of environmental security, and then explains four major categories of environmental security problems—namely, the way environmental change can be a factor in violent conflict, the way environmental change can be a risk to national security, the way war and preparation for war can damage the environment, and the way environmental change can be a risk to human security. It explains how environmental security can mean different things to different people and can apply to vastly different referent objects in ways that sometimes have very little to do with environmental change.