Anovulatory infertility, affecting 40% of infertile women, is classified into three groups according to the World Health Organization (WHO). WHO Group 1 consists of patients with hypogonadotropic hypogonadism, WHO group 2 involves normogonadotropic anovulation, which mainly refers to polycystic ovary syndrome (PCOS), and in WHO group 3 are patients with hypergonadotropic hypogonadism. In hypogonadotropic hypogonadism blood levels of gonadotropins and estradiol are very low due to congenital (such as Kallmann syndrome), acquired (tumors, infiltrative diseases, infections, trauma, and vascular disorders), and functional (functional hypothalamic amenorrhea) causes. In PCOS the diagnostic criteria of Rotterdam ESHRE/ASRM consensus are valid with modification of the ulatrasonographic evaluation. In PCOS there are significant aberrations both at pre-antral and antral stages of folliculogenesis, due to hyperandrogenism, hypersecretion of LH, and inappropriate expression of AMH, insulin-like growth factors, oocyte-derived factors (such as bone morphogenetic proteins), and microRNAs. In hypergonadotropic hypogonadism there are very high levels of gonadotropins and very low levels of estradiol in the circulation mainly due to premature ovarian insufficiency (POI). POI may occur due to a reduced pool of oocytes, accelerated follicular atresia or impaired folliculogenesis, while the causes of POI include chromosomal and genetic defects, autoimmune disorders, chemotherapy, radiation, infections, and surgery, although many cases may be idiopathic.