This book explores the crucial concept of responsibility and the critical role it plays in Raymond Aron’s political philosophy. This book argues that it is a key concept that is found throughout Aron’s political thinking. Aron learned to appreciate the importance of responsibility from several authors; most notably, Max Weber and von Clausewitz and it is present throughout many of his writings. The concept is found in Aron’s discussions of war and peace, ideology and totalitarianism, and on freedom. The book also discusses Aron’s political legacy and suggests that his political thinking can help us address many of the issues of the 21st century.