The following factors are involved in the ovarian cycle (i.e., follicular development, ovulation, and corpus luteum formation) and the regulation of menstruation as a result of luteal regression. All these factors are so- called “classical hormones” that are secreted by specific cells or organs. These factors affect target organs via the bloodstream and constitute the core of the feedback mechanism of the diencephalon (hypothalamus)– pituitary–ovary–uterus system. A. Gonadotropin-releasing hormone (GnRH); B. Gonadotropins: follicle- stimulating hormone (FSH) and luteinizing hormone (LH); C. Steroid hormones: estrogen, androgens, progesterone, glucocorticoids, and mineralocorticoids; and D. Glycoproteins: inhibin, activin, follistatin, and anti-Müllerian hormone (AMH). In this chapter, particularly important aspects of hormones A–D are selected and described. The menstrual cycle is controlled via regulation of GnRH, which is synthesized by the hypothalamus. GnRH, a polypeptide composed of 10 amino acids (Glu-His-Trp-Ser-Tyr-Gly-Leu-ArgPro-Gly- NH2), is a hormone that is secreted by the hypothalamus and that has an extremely short half-life (2–3 min). The GnRH-1 gene (GNRH1) is located on human chromosome 8 (8p11.2-p21) and produces a 92-amino acid precursor peptide called prepro-GnRH, which includes a signal sequence (23 amino acids), GnRH (10 amino acids), a proteolytic processing site (3 amino acids), and GnRH-associated peptide (56 amino acids).