Despite their name, sex hormones are not currently very sexy. Compared to genes, chromosomes and stem cells, they seem rather old-fashioned biological actors. Unlike genes and stem cells, no major funding initiatives for the British social sciences focus on their actions. In many ways, hormones are quintessentially twentieth-century entities: ‘born’ in a time when scientific biology was very new, genetics hardly considered and stem cells only just ‘discovered’, they had their glory days in the mid to late century, gradually becoming mundane and even troublesome in more recent years. Hormonal action today is increasingly difficult to grasp as scientists and clinicians learn more about hormones, wonder about how even to define them as a group and raise serious questions about their safety as medications. Despite these difficulties, hormones have demonstrated great tenacity in remaining at the centre of explanations of ‘life’ and particularly of sex and sexual differences. In all kinds of discourses – technoscientific, biomedical, social scientific, popular, media – and in everyday conversation, hormones perform important explanatory work in descriptions of who we are and how we reproduce. They are active in bodies of all sorts; as endogenous chemicals, medications and environmental toxins. Sex hormones today surround us, affecting humans and other animals in barely perceptible ways, altering, it is claimed, what our futures might hold. Hormones and chemicals that act like them breach all kinds of boundaries: between species (creating polar-bear–human hybrids), places (the Arctic is suddenly near England), times (the effects of toxic chemicals are transgenerational) and sexes (males are demasculinised and feminised, females masculinised and defeminised).