The text encompasses the basic science of reactive oxygen species (ROS) production by mammalian spermatozoa, the way in which these highly reactive molecules are processed by the germ line and the physiological significance of this redox activity in the generation of a functional gamete. The factors responsible for perturbing the delicate balance between physiological redox signaling on the one hand and oxidative stress on the other are also extensively reviewed and some of the first clues concerning the underlying mechanisms (age, heat, infection, cryostorage, aberrant lipid metabolism), clearly identified. From a clinical perspective there are chapters setting out the methods we should be using to diagnose oxidative stress in the male germ line, a clinical perspective on the aetiology of this condition and detailed considerations of the most suitable means of ameliorating such stress from a therapeutic point of view.