Gonadotropin-releasing hormone (GnRH) is produced and released from group of loosely connected neurons located in the medial basal hypothalamus. The GnRH pulse frequency varies throughout the menstrual cycle with pulses occurring less frequently in the luteal phase. Any change in the frequency or amplitude of the GnRH pulses causes disarray in the menstrual cycle. GnRH agonists were developed in order to put GnRH into clinical use. Substitution of glycine at number 10 at the C-terminus was the first major development leading to the increase in the potency of GnRH. Administration routes of GnRH agonists are intramuscular or subcutaneous depot injection, or daily intranasal or subcutaneous administration. The most commonly used GnRH agonist protocols during controlled ovarian hyperstimulation are defined in this chapter. A detailed discussion on GnRH antagonists and clinical uses of GnRH analogs has been also provided in this chapter.