A comprehensive guide to the history and current shape of conspiracy theories in American life, this work provides vital context for understanding scientific research and findings about the origins, type, function, and appeal of conspiracy theories. It also serves as a scholarly but accessible overview of prominent conspiracy theories, past and present, and their enduring impact on American life and culture. A valuable resource for exploring and understanding the fascinating subject of conspiracy theories, this work explains the origins and evolution of byzantine, outlandish, and often malicious theories that have echoed down through the years, from the ones that continue to swirl around the assassination of President John F. Kennedy to others with deep roots in anti-Semitism, racism, and sexism. It places a particular focus, however, on contemporary social-media fueled theories related to voting practices and elections; school shootings; the September 11 attacks; the alleged existence of diabolical and power-hungry secret societies; and scientific findings about vaccines, climate change, and other high-profile issues.