While the number of think tanks active in American politics has more than quadrupled since the 1970s, their influence has not expanded proportionally. Instead, the known ideological proclivities of many, especially newer think tanks with their aggressive efforts to obtain high profiles, have come to undermine the credibility with which experts and expertise are generally viewed by public officials. This book explains this paradox. The analysis is based on 135 in-depth interviews with officials at think tanks and those in the policy making and funding organizations that draw upon and support their work. The book reports on results from an original survey of congressional staff and journalists and detailed case studies of the role of experts in health care and telecommunications reform debates in the 1990s and tax reduction in 2001.