The first history of polyamory, this work examines the roots of sexual non-monogamy in political thought and countercultural spiritualism and traces its path to mainstream practice and cultural discussion today. Recent studies have found that as many as one in five Americans have experimented with some form of sexual non-monogamy, and approximately one in fifteen knows someone who was or is polyamorous. Although gathering statistics on polyamorous people is challenging, there has clearly been a growing interest in and normalization of relationship practices defined by emotional intimacy and romantic love among multiple people. Over the past decade, the mainstream media has increasingly covered polyamorous lifestyles and the committed relationships of throuples, and popular dating apps have added polyamory as a status option. This book is the first history to trace the evolution of polyamorous thought and practice within the broader context of American culture. Drawing on personal journals and letters, underground newsletters, and publications from the Kinsey Institute Archives, among other sources, it reconstructs polyamory's intellectual foundations, highlighting its unique blend of conservative political thought and countercultural spiritualism. Christopher M. Gleason locates its early foundations in the Roaring Twenties among bohemians. In the 1950s and 1960s it surprisingly emerged among libertarian science fiction writers. The next wave of polyamorists belonged to countercultural communities that rejected traditional Christianity; some were neo-Pagans and New Age tantric practitioners who saw polyamory as intrinsic to their spirituality. During the 1980s polyamory developed as a coherent concept, faced backlash from conservatives, and tried to organize into a social and political movement with a national network. Throughout the 1990s, polyamorists utilized the internet to spread their ideas, often undermining any remaining religious or spiritual significance their ideas held. Polyamory now encompasses a diverse set of people, from those with libertarian leanings seeking unlimited freedom from government interference in their consensual partnerings to those who pursue legal recognition of their relationships, especially issues revolving around children. Offering an original perspective on sexuality, marriage, and the family, American Poly reveals the history of polyamory in the United States from fringe practice to a new stage of the sexual revolution.