In the late-nineteenth and early-twentieth centuries, a broad community of radical men and women engaged in discussions about sex reform and what they termed ‘free love’. Much of this debate took place within a particular community of periodicals, as those interested in radical sexual reform read, contributed to, and corresponded with a small number of key sex radical journals such as The Adult, Lucifer, the Light-Bearer, and The Freewoman. Drawing upon their contributions to these journals, this thesis will examine the ways in which sex radical authors built and shaped their beliefs about sex and sex reform – in short, how they constructed ‘free love’ in their work. In particular my research will explore how sex radicals, despite holding diverse and often conflicting views, used similar theories and ideas drawn from a broad range of scientific disciplines to support their arguments. This thesis will show that radicals used a varied set of scientific ideas and theories in order to contend that mankind had a ‘natural’ and important sexuality that had been harmfully bound and distorted by contemporary social, cultural, and legal institutions. It will demonstrate that it was these scientific ideas that underpinned their criticisms of existing social institutions, and thus framed their varied calls for radical sexual reform. Despite the often contentious nature of sex radical debates, this thesis will therefore illustrate that radical authors throughout these journals shared a belief that a scientific understanding of sex was crucial to making sex ‘free’. Furthermore, by exploring links between sex radicals and other social reformers, research will illustrate that radicals were not isolated and should not be dismissed as a marginal group; instead it will show that they are better understood as active participants in part in a broad set of contemporary intellectual debates about issues related to sex, relationships, gender, and the body. As such, this thesis will show the importance of bringing radicals in from the fringe of historical accounts in order to gain a more in-depth understanding of such debates.