Evidence is presented to link components of the metabolic syndrome to testosterone deficiency and obesity. Testosterone deficiency in hypogonadism or testosterone deprivation in normo-gonadotropic men increases fat mass as well as fasting insulin levels. Testosterone supplementation (TS) in a dose dependent manner, increase lean body mass (LBM), reduces fat mass, body mass index (BMI) and waist hip ratio in both young and elderly hypogonadal men. A negative association between T and insulin resistance as well as impaired glucose intolerance has been demonstrated and in type 2 diabetic men TS improves metabolic parameters. TS improves most components of the metabolic syndrome and also reduces inflammatory cytokines.