While members of most sexually reproducing species are either male or female, individuals belonging to the rest of the biological world are not so simply defined. Hermaphroditic creatures reproduce both as a male and as a female, providing a fascinating example of alternative sexual practice and its ecological and evolutionary successes. Eloquently written by an award-winning biologist and pioneer in molecular ecology, this primer traces the phenomenon of hermaphroditism throughout Earth's species and the adaptive significance of alternative sexual systems. Accessible and richly illustrated, the text maps the evolutionary origins of hermaphroditism, as well as its historical instances and fictional representations, underscoring the relevance of dual sexuality to our biological, intellectual, and cultural making.