Existing research on hermaphroditism in Europe has centred on France, Germany and England and research on the Spanish case has mostly been confined to the sixteenth and eighteenth centuries. This study seeks to analyse how discourse on hermaphroditism in Spain may be recast by means of a strong comparative European approach centred on the nineteenth and twentieth centuries. Spain, despite following broad European trends in sexual science, is posited not as a country that merely reproduced others' ideas about hermaphroditism but, from a dynamic and comparative methodological perspective, as a country in which scientists actively engaged with imported theories, elaborating local understandings of their own making. This book traces how the hermaphrodite was conceptualized within a changing and increasingly medicalized environment that shifted from an emphasis on the defining model of the gonads to the development of endocrinology and psychological gender science.