Overweight and obesity are pandemic problems, occurring in high-, middle-, and low-income countries (particularly in urban areas), in both sexes, and in all age groups. Obesity can cause changes in the levels of sex hormones, in the spermatogenic process, and in sperm maturation, leading to reduced sperm quality, oligospermia, damage to DNA integrity, affecting sperm motility and capacitation, and therefore interfering in the fertilization process and in the quality of the embryo. Moreover, the increase in adipose tissue may cause elevated concentrations of sex steroids, adipokines, and leptin, as well as the aromatization of androgens into estrogens, which may accelerate the onset of puberty and, subsequently, the seminal quality and homeostasis of sex hormones in adulthood. However, the literature is contradictory about the effects of obesity on the onset of puberty, sperm and male fertility.