In what circumstances, if any, would the recourse to war be justified? What restraints should be binding on any such use of force? The pre-eminent framework for addressing these questions is the just war tradition. Boasting a history that can be traced back to ancient Rome, this corpus has been distilled over time into a set of principles that bear on two poles, the jus ad bellum, the norms that govern the resort to war, and the jus in bello, the constraints binding on its prosecution. Scholars dispute the composition of these poles, but all concur that the former turns on “just cause,” “proper authority,” and “right intention,” while the latter includes “proportionality” and “discrimination.” This chapter discusses the prizes and pitfalls that attend the study of the ethics of war in light of the historical just war tradition.