Polycystic ovary syndrome (PCOS) is a syndrome with varied manifestations both within different populations and between different populations. With recent increases in the understanding of the pathophysiology of PCOS and the recognition of the importance of ultrasound in defining the morphology of the polycystic ovary, the syndrome has now been defined as the presence of two of the following three criteria: oligo-ovulation and/or anovulation, hyperandrogenism (clinical and/or biochemical), polycystic ovaries; with the exclusion of other aetiologies of menstrual disturbance and androgen excess. There are likely to be many routes to the development of PCOS, including genetic predisposition, environmental factors and disturbances of a number of endocrine pathways. To establish the diagnosis of PCOS it is important to exclude other disorders with a similar clinical presentation, such as congenital adrenal hyperplasia, Cushing syndrome, and androgen-secreting tumours.