This chapter examines war and military participation. It addresses losses of human life among combatants and the civilian population, as well as long-term demographic consequences of such losses. The chapter also examines the effects of military conflict on physical and mental health of the population. Since the 1700s, the military has been a formal institution orchestrated by a government. Military service is often characterized as voluntary with periods of involuntary or mandatory service depending on the situation and nation-state in context. While characterized constitutionally as an all-volunteer force, the United Stated has used conscription or the draft to fill ranks during major conflicts, including the American Civil War, both World Wars, and the wars in Korea and Vietnam. While this military service has often focused primarily on men serving, as of 2013, some countries began to conscript or draft women into service.