This book describes militias as significant and prevalent actors in today's international security environment. To separate them from other types of violent non-state groups, such as terrorists, guerrillas and insurgents, the author describes militias as local guardians that use violence to fill a variety of political, social and security gaps, which have created vulnerabilities for their particular constituencies.  Although militias are local in orientation, their effects are not contained to particular countries and have only added to the instability in the international system. This book explores how militias contribute to international security issues by furthering state fragility, undermining human rights and democratization, enabling illicit trafficking, prolonging internal conflicts and fostering proxy wars.