Over the course of history, many cross-cultural efforts have been made to understand better the form and function of male same-sex behaviour. Initial naming exercises evaluated the sexual actions taken, and categorized these behaviours as expressions of crime, sin and disease. Various historical accounts note that it was in fin-de-siècle Germany and England, however, that several concepts were developed for the first time to encapsulate male same-sex behaviour, and to identify the type of men engaging in such conduct, in a more tolerant way. Operating within the taxonomic impulse of the eighteenth century, sexology — the scientific study of sexualities and sexual preferences that were considered to be unusual, rare, or marginalized — spurred the development of these new concepts. In the aim of better understanding humans through scientifically evaluating, quantifying, and labelling their sexual form and function, sexology moved male same-sex behaviour beyond the notions of crime, sin and disease. This thesis argues that the key works of sexologists Karl Heinrich Ulrichs (1825-1895), Károly Mária Kertbeny (1824-1882), Henry Havelock Ellis (1859-1939) and Magnus Hirschfeld (1868-1935) were instrumental to the theoretical endeavour of reclassifying male same-sex behaviour. These four sexologists operated within the parameters of what Foucault calls scientia sexualis: the machinery needed for producing the truth of sex via confessional testimony. Through their own confessional testimony, and testimony collected from other men with same-sex behaviour, Ulrichs, Kertbeny, Ellis and Hirschfeld deemed same-sex behaviour to be a phenomenon based on congenital conditions and one which manifested itself in the form of an inherent sex/gender misalignment. While this behaviour was uncommon, it was not abnormal due to its biological origin. Same-sex behaviour was simply an anomaly of sorts — one specific and rare form of attraction on a spectrum of possibilities. This rationalization of same-sex behaviour differed greatly from the work of other sexologists of the time who evaluated same-sex behaviour to be symptomatic of crime, sin and disease like degeneration theorist Richard von Krafft-Ebing. In arguing that same-sex behaviour developed naturally prior to birth, Ulrichs, Kertbeny, Ellis and Hirschfeld empowered men with same-sex behaviour to negotiate new identities for themselves outside of crime, sin and disease. This discursive rebranding of same-sex behaviour is an example of what feminist postructuralism labels as reverse discourse. In order to negotiate new identities for themselves and others with congenital same-sex behaviour, Ulrichs, Kertbeny, Ellis and Hirschfeld developed four specific concepts. These terms are: Urning (1865), homosexualität (1869), sexual inversion (1897), and third sex (1914). While these examples of reverse discourse were operationalized within restrictive conceptualizations of gender expression, they moved away from classifying same-sex behaviour as temporary acts to classifying those engaging in this behaviour as a specific species of people. This transition from sexual act to personage has been elaborated upon most famously by Michel Foucault in The History of Sexuality, Volume 1 (1978/1990).