How doctors decide whether someone has PCOS, and why it shows up so differently from one woman to the next. The chapter walks through the history of the diagnostic criteria and the Rotterdam Criteria, under which PCOS is diagnosed when two of three features are present: irregular periods, raised male hormones and polycystic ovaries on ultrasound. It then explains the four Rotterdam phenotypes, how their spread differs between countries, and the two newer subtypes, reproductive and metabolic, including which populations in Asia and Singapore each is more common in and what that means for fertility.