Polycystic ovarian morphology is seen on ultrasound in approximately 22% of women. Polycystic ovary syndrome (PCOS) is a common and perplexing endocrine disorder of women in their reproductive years, with a prevalence of up to 10%. Clinical expression of the syndrome varies but commonly includes menstrual cycle disturbance, hyperandrogenism, insulin resistance and obesity. Recently, the European Society for Human Reproduction and Embryology and the American Society for Reproductive Medicine (ESHRE/ASRM) achieved a new consensus regarding the definition of PCOS. This is now defined as the presence of any two of the following three criteria: (i) polycystic ovaries; (ii) oligo-/anovulation; and/or (iii) clinical or biochemical evidence of hyperandrogenism. This revised definition provides an international framework for the clinical assessment of PCOS and for future research and collaboration.