Whether called friendly fire, fratricide, or amicicide, the problem of engaging one’s own forces on the battlefield is an old one. Long familiar to soldiers and students of military affairs, friendly fire became a hot public issue only during the Persian Gulf War of 1991, when 35 U.S. soldiers and marines were killed and 72 were wounded in 28 friendly fire incidents. Just more than 23% of all U.S. personnel casualties and about 77% of all U.S. combat vehicle losses in the Gulf were due to friendly fire, and other coalition forces suffered similar losses. This entry discusses aspects of friendly fire such as accountability, costs, causes, and prevention.