Polycystic ovary syndrome (PCOS) has reproductive and metabolic implications. Insulin resistance (IR), secondary to genetic and lifestyle factors, is integral in the pathogenesis, metabolic, clinical features and the long-term sequelae in the majority of people with PCOS. Therapeutic strategies targeting IR in PCOS ameliorate clinical features and might reduce long-term sequelae including diabetes. The mainstay for improving IR is lifestyle change; however, feasibility and sustainability remain concerns. In PCOS, metformin reduces IR, improves ovarian function, regulates cycles, lowers androgens, improves clinical hyperandrogenism and potentially improves fertility. Metformin is also likely to delay diabetes onset and has a role in PCOS in those at high risk of diabetes; however, further research is needed to clarify specific target subgroups and clinical indications.