This chapter explains how PCOS affects the menstrual cycle and ovulation. It begins with what a typical cycle looks like, then explains why irregular cycles are one of the three main diagnostic criteria for PCOS, alongside polycystic ovaries and high androgens. It sets out how the Rotterdam Criteria define irregular ovulation, and why the ovaries in PCOS often contain many small follicles that do not mature, leading to anovulation and missed periods. It finishes with the four PCOS phenotypes, A to D, and how ovulation patterns can differ between them, which helps explain why experiences of PCOS vary.