The polycystic ovary syndrome (PCOS) is the most frequent endocrine disease in women of reproductive age. Hyperandrogenism, anovulation and metabolic syndrome are the cardinal features of PCOS. Hyperandrogenism results from a diffuse enzymatic hyperactivity at the theca-interstitial cell level. Anovulation is due to an impairment of the selection of a dominant follicle, while the number of smaller follicles is exaggerated. The molecular grounds of insulin resistance could be an increased Serine phosphorylation of the insulin receptor. The clinical classification of PCOS distinguishes three forms: the classic PCOS, where the three above mentioned features are present, the non classic PCOS and the asymptomatic PCOS, revealed by ultrasonography. Only the increased ovarian volume or surface (>11ml and> 5.5cm(2), respectively) must be viewed as a specific ultrasonic sign of PCOS. Cyproterone acetate remains the basic treatment of hyperandrogenism. The treatment of anovulation and infertility follows a consensual strategy. The insulin sensitizing treatment allows to decrease hyperandrogenism, to reverse the menstrual cycle irregularity and to obtain spontaneous or induced pregnancies.