The tension between power and principle has long been central to international legal discourse. Instrumentalism – the idea that international law can change behaviour, but only by creating constraints and opportunities that affect state interests – straddles this tension. This chapter chronicles the rise, role and contestation of instrumentalism in international legal thought. On its own terms, instrumentalism is a descriptive tool to explain how power and norms interact to shape state behaviour. Critics, however, worry that instrumentalism legitimates power politics by clothing them in the trappings of principled legal arguments. Arguments that international law is not ‘law’ or is widely ineffective – arguments that critics fear are aimed at delegitimizing international law as a normative system – have fuelled these fears. The chapter argues that instrumentalist thinking has largely survived these critiques. Instrumentalism continues to flourish in both academic work and international legal practice, in large part because its usefulness as a descriptive tool has been used to understand how to make international law more effective.