Many cross-sectional analyses and longitudinal studies have examined the association between adiposity and pubertal development. In addition, the impact of an increased fat mass on reproduction and fertility in human obese men and in male animal models of obesity has been studied. A trend toward earlier pubertal development and maturation in both sexes has been shown, and the notion that obese boys might progress to puberty at a slower pace than their nonobese peers can no longer be substantiated. Impaired fertility markers and reduced reproductive functions have been observed in obesity. Obesity affects both pubertal development and fertility in men.