GnRH analogues were introduced into clinical practice more than 20 years ago. The recent development of new GnRH antagonists (GnRHant) has revolutionized the treatment protocols of infertility. Ovarian stimulation has become easier, safer and more convenient for patients. This review presents data on the physiological background of GnRH and its analogues--agonists as well as antagonists--and highlights the different aspects of the clinical use of these substances. Two protocols, the single dose and multiple dose protocol are available and lead to comparable results. Data from prospective, randomized trials as well as meta-analyses suggest, that a more tailored, individualized approach concerning the day of administration of GnRHant may lead to more favorable results with regard to ovarian response, implantation and pregnancy rates. Possible negative effects of GnRHant, mainly based on in vitro models are discussed. The outcome of children born after GnRHant use has shown, that this medication is safe. On the whole, the use of GnRHant is advantageous as compared to the traditional way of ovarian stimulation, i.e., the GnRH agonist long protocol. However, especially the effectiveness of these compounds will have to be clarified in the near future by carefully designed prospective, randomized studies.