Polycystic ovary syndrome (PCOS) is a highly prevalent disorder, representing the single most common endocrine–metabolic disorder in reproductive-aged women. Currently there are four recognized phenotypes of PCOS: 1) hyperandrogenism+oligo-anovulation+polycystic ovarian morphology; 2) hyperandrogenism+oligo-anovulation; 3) hyperandrogenism+polycystic ovarian morphology; and 4) oligo-anovulation+polycystic ovarian morphology, each with different long-term health and metabolic implications. Clinicians should clearly denote a patient's phenotype when making the diagnosis of PCOS. Polycystic ovary syndrome is a highly inherited complex polygenic, multifactorial disorder. Pathophysiologically abnormalities in gonadotropin secretion or action, ovarian folliculogenesis, steroidogenesis, insulin secretion or action, and adipose tissue function, among others, have been described in PCOS. Women with PCOS are at increased risk for glucose intolerance and type 2 diabetes mellitus; hepatic steatosis and metabolic syndrome; hypertension, dyslipidemia, vascular thrombosis, cerebrovascular accidents, and possibly cardiovascular events; subfertility and obstetric complications; endometrial atypia or carcinoma, and possibly ovarian malignancy; and mood and psychosexual disorders. The evaluation of patients suspected of having PCOS includes a thorough history and physical examination, assessment for the presence of hirsutism, ovarian ultrasonography, and hormonal testing to confirm hyperandrogenism and oligo-anovulation as needed and to exclude similar or mimicking disorders. Therapeutic decisions in PCOS depend on the patients' phenotype, concerns, and goals, and should focus on 1) suppressing and counteracting androgen secretion and action, 2) improving metabolic status, and 3) improving fertility. However, despite significant progress in understanding the pathophysiology and diagnosis of the disorder over the past 20 years, the disorder remains underdiagnosed and misunderstood by many practitioners.