This Element surveys the field of defense, peace, and war economics with particular emphasis on the contributions made by Austrian economists. I first review treatments of defense, peace, and war by the classical economists. I then discuss the rise of a distinct and systematic defense, peace, and war economics field of study starting in the 1960s. Next, I consider the contributions by Austrian economists to the field. This includes the economic analysis of the nature of the war economy, problems with the public good justification for the state-provision of defense, the seen and unseen costs of war, the idea of the liberal peace, and the realities and limitations of foreign intervention. I conclude with a discussion of some open areas for future research.