After the success of relief efforts to the displaced Kurdish population in northern Iraq following the Gulf War, many in the US military and the international relief community saw military forces as critical partners in the response to future complex emergencies (CEs). However, successes in subsequent military involvement in Somalia, Rwanda, the former Yugoslavia, and other CEs proved more elusive and raised many difficult issues. A review of these operations reinforces some basic lessons that must be heeded if the use of military forces in humanitarian relief is to be successful. Each CE is unique, thus, each military mission must be clearly defined and articulated. Armed forces struggle to provide both security and humanitarian relief, particularly when aggressive peace enforcement is required. Significant political and public support is necessary for military involvement and success. Military forces cannot execute humanitarian assistance missions on an ad hoc basis, but must continue to develop doctrine, policy and procedures in this area and adequately train, supply, and equip the units that will be involved in humanitarian relief. Militaries not only must cooperate and coordinate extensively with each other, but also with the governmental and non-governmental humanitarian relief organizations that will be engaged for the long term.