The adipose tissue is an essential endocrine organ. The adipokines produced by this tissue, which include leptin, adiponectin, apelin, visfatin and resistin, regulate many body functions. Being an endogenous regulator and modulator of the male and female reproductive systems, leptin acts on all components of the hypothalamic-pituitary-gonad axis. It stimulates the secretion of gonadoliberin by the hypothalamic neurons and gonadotropins by the pituitary gonadotrophs, increases the sensitivity of gonadotrophs to gonadoliberin, and affects the steroidogenesis, spermatogenesis and folliculogenesis. The effect of leptin on the gonads is dose-dependent. At the physiological concentrations, leptin stimulates the steroidogenesis, ovulation and receptivity of the endometrium, and at the high concentrations, leptin suppresses them. The prevalence of infertile marriages is growing steadily, and the assisted reproductive technologies, especially the in vitro fertilization (IVF) are becoming more widespread. Leptin is a promising marker in preparing for IVF cycles, since in patients with successful IVF outcomes, the leptin levels in the blood and follicular fluid is lower than in patients with unsuccessful IVF outcomes. Along with this, leptin increases sperm fertility and can be used for sperm cryopreservation.