The balance of power principle has been central to both the study and practice of international politics for over 300 years. It has guided governments in the conduct of foreign policy and provided a structure for explanations of some of the recurring patterns of international relations. This study examines the various meanings given to the balance of power over the centuries and traces the historical evolution of its theory and practice through steadily more complex forms. It describes the balance principle in practice, both as a guiding light of national foreign policies and as a structural explanation of how the international system operates. The reader is provided with an understanding of the various meanings of the balance principle and the key thinkers and politicians who have influenced its development. The text presents the essence of arguments concerning the morality of the principle as a foreign policy guide and its value as a structural explanation of the fundamental reality of international relations.