The effectiveness of a state's claim to territory has long been one of international law's central concerns when determining territorial entitlement. However the concept has always been the subject of controversy, as states and empires have adapted the notion over the centuries to suit their acquisitive ambitions, sometimes using it to challenge their rivals' exorbitant territorial pretensions, and other times invoking it to validate their own seizure of territory by unlawful means. In the last century, as the international legal system has undergone profound changes in its structure and substance, arguments concerning the effectiveness of a state's exercise have continued to feature prominently in practice, and the evidences of a state's display of authority, known as effectivites, appear unfailingly in territorial cases before international courts and tribunals. And yet there remain crucial uncertainties about the notion. What exactly constitutes effectiveness? And what legal consequences can it produce? This book seeks to clarify the content, character and legal significance of the principle of effectiveness in the international law of territory. By surveying the diplomatic practice and reviewing the extensive international case law, it demonstrates that effectiveness remains a vital element of the international law of territory, though its meaning has evolved and its functions have multiplied. It also shows that, while effectivites have become a ubiquitous element in modern territorial litigation, their treatment has been far from uniform. Through a systematic analysis of the various ways in which effectivites have been used in territorial disputes, this book reveals the dynamic and complex manner in which the principle of effectiveness interacts with other shifting elements of international law concerning territory, including the prohibition on the use of force, uti possidetis, the law of treaties, inter-temporality and the critical date, self-determination and international legal personality. With its distinctive analysis of the theoretical debates, historical practice and modern case law, this book sheds new light on the adjudication of disputes over territorial sovereignty and land boundaries today.