The ageing process in men is marked by changes in body composition (loss of skeletal muscle and gain in fat mass) and is associated with a decline in serum testosterone; this decline is augmented in obese men. Correlations between these aspects of ageing, and the acknowledged role of exogenous testosterone in reversing the changes in body composition seen in adult men with congenital or acquired hypoandrogenism, has led to the hypothesis that testosterone therapy will result in favourable changes in body composition and may improve metabolic status and / or cardiovascular risk, particularly in obese men. Data from randomized controlled trials of testosterone therapy in middle-aged and older men addressing the endpoints of body composition consistently document a beneficial response, with the magnitude of change related to baseline fat mass and serum testosterone levels. The effects on metabolic parametres and ultimately on cardio-metabolic outcomes is less well documented.