Economic analysis and rational choice have made significant inroads into the study of international law and institutions in the last decade, relying upon standard assumptions of perfect rationality of states and decisionmakers. This approach is inadequate, both empirically and in its tendency toward outdated formulations of political theory. This Article presents an alternative behavioral approach that provides new hypotheses addressing problems in international law while introducing empirically grounded concepts of real, observed rationality. First, I address methodological objections to behavioral analysis of international law: the focus of behavioral research on the individual, the empirical foundations of behavioral economics, and behavioral analysis’s relative lack of parsimony. I then offer indicative behavioral research frameworks for three contemporary puzzles in international law: (a) the relative inefficiency of the development of international law, (b) dissent in international tribunals, and (c) target selection in armed conflict. Behavioral research in international law can serve as a viable, enriching alternative and complement to conventional economic analysis, so long as it is pursued with academic and empirical rigor as well as intellectual humility.