摘要
Think tanks as an important force in the policy making field in the United States have strengthened their visibility and expanded their influence in recent decades. This paper contends that the historical and political climate of the United States has fostered the development of think tanks and they will continue to exert their influence in the policy making field of the United States. It mainly examines the history of American think tanks and the roles they have been performing during more than one hundred years＇ evolution. In the twenty-first century, in a more and more flattering world, policy issues become increasingly complicated, therefore think tanks are indeed fixtures of American policy making scene. They play six major roles （1） source for policy idea; （2） venue for communicating and initiating new policy ideas; （3） source for personnel; （4） educating and informing the public; （5） evaluator of government policies; and （6） Track lI diplomacy.
