GnRH agonists and antagonists are a group of synthetic compounds resulting from natural GnRH through amino acid substitution on different positions. They have greater biological stability and a more increased affinity for the receptors than natural GnRH. GnRH agonists initially increase gonadotropin secretion from pituitary cells, known as the flair-up effect, followed by suppression of pituitary function and gonadotropin secretion due to down-regulation of GnRH receptors. GnRH antagonists bind competitively to the receptors and prevent the endogenous GnRH from exerting its stimulatory effect on the pituitary cells, thus avoiding any flare-up effect. The main purpose of GnRH analogs use in ovarian stimulation is the prevention of untimely LH surges. GnRH agonists can be administrated according to the ultra-short, short and long protocols. The use of GnRH agonists in cycles stimulated for assisted reproductive techniques (ART) is associated with the significant decrease in the cancellation rates, an increase in the number of oocytes and embryos obtained and higher pregnancy rates. Follicle cyst formation is an undesired side effect of agonist administration when long protocol is used. OHSS is the most serious side effect of the ovarian stimulation and its occurence cannot be avoided by GnRH agonist administration. Antagonist protocols have clear advantages, such as a shorter stimulation period, no sexual steroid withdrawal symptoms and significantly lower rate of OHSS. Based on recent results, pregnancy rate after IVF does not depend on the type of applied analogs in ovarian stimulation protocols. Other indications of GnRH analogs in human reproduction are for treatment of endometriosis and uterine fibroids.