Polycystic ovary syndrome (PCOS) is the most frequent endocrine disease among women with childbearing potential, the best-known cause of hirsutism, with a hypothesized prevalence of 8-22%. The first part of the paper discusses the conceptional evolution of the syndrome, from its description in 1935 by Stein and Leventhal till today. It describes the changes in the criteria systems, emphasizing that the Rotterdam criteria, proposed in 2003 by the European Society for Human Reproduction and Embryology/American Society for Reproductive Medicine, are still valid today. This system basically differs from earlier (1990) NIH-criteria in one aspect: it introduced two newer phenotypes, one without hyperandrogenism and the other with ovulatory cycles, so it distinguishes 4 phenotypes. The etiology and pathogenesis of PCOS is heterogeneous, multifactorial, poorly understood. We present the 3 leading hypotheses (1 - hypothalamo-hypophyseal disturbances, 2 – primary enzyme disorders in ovarian, or ovarian/adrenal steroidogenesis, resulting primarily in hyperactivity of 17alpha-hydroxylase/17,20-lyase, 3 – insulin resistance-hyperinsulinism and other metabolic dysfunctions). We emphasize the role of genetically determined hyperandrogenism, that of insulin resistance-hyperinsulinism and the importance of reinforcing each other. Subsequently, the aggravating aspects of the frequently associated metabolic syndrome are discussed, and then the effects of the mentioned pathological processes on the endocrine and other organ structures participating in the regulation of sexual functions. We stress the hypothetical role of perinatal and pubertal androgen exposition in the pathogenesis of PCOS. The mechanisms of anovulation and those of the endometrial lesions are discussed, too. The clinical manifestations, the paraclinical and laboratory examinations, the positive and differential diagnosis and the complications are also presented. We intend to deal with the therapeutic aspects of PCOS in an upcoming paper.