This chapter examines the role of testosterone in the development of the physical and mental health of the male from prenatal development to adulthood. We will examine the biological pathways of testosterone and how these impact psychology (e.g. mental rotation ability) and psychopathology (e.g. autism). We explore the role of testosterone in the adult male, its relationship with traits associated with masculinity and assess whether it is fair to say that testosterone causes aggression in humans. We conclude that the view of testosterone in the social sciences needs to be refined and updated in order to recognise the importance of this sex hormone to the mental and physical health and wellbeing of males.