Public policy science is issue-oriented, focused on problem-solving and attempts to identify society's objective problems and provides possible solutions to them. Policy makers have various tools of decision-taking from which Think Tank is the main. Think Tank is an independent and non-profit research organization that evaluates and provides solution in a broad range of issues. Think Tanks are traditionally considered as independent voices of policy issues in domestic and foreign policy which conduct scholarly research and determined to provide policy makers with their expert knowledge. However, the governance acts as fertile situation for growing the numbers of Think Tanks. Governing from the government's perspective is always an interactive process because no single actors, public or private, has the knowledge and resource capacity to tackle problems unilaterally. This article attempts to review the history of Think Tanks, The various kinds of think Tanks, their role and impact in policy process and their influence channels on policy makers. Then, this deals with that why these organizations have great influence and growing numbers in the United States of America.