Since the 1950s, the right to conscientious objection to military service in international human rights law has excited the interest of both non-governmental and inter-governmental organizations in a variety of contexts. This book examines the subject, beginning with an exploration of the concept of conscience and its evolution with a view toward understanding the meaning and potential scope of the right to conscientious objection from a legal perspective. It also describes the different categories into which conscientious objectors can be divided, explain the differences between these categories, and investigate how these differences are interpreted at national and international levels. It then investigates the right to conscientious objection as a legitimate exercise of freedom of thought, conscience, and religion in international human rights law. In this regard, this book deeply analyzes human rights law at both the international and regional level, examining UN, European, and Inter-American mechanisms.