Insulin-sensitising agents are frequently used in the treatment of women with polycystic ovary syndrome (PCOS). This chapter explores the use of insulin sensitisers, primarily metformin, for varying indications related to PCOS and discusses the evidence to develop a risk/benefit ratio for their use. These drugs were developed to treat type 2 diabetes and have been adapted as treatments for the symptoms of PCOS. Metformin has been proposed to prevent early first-trimester miscarriage. Randomised trials, primarily from one group in Spain, have shown that metformin improves many aspects of premature pubarche, including slowing the onset of puberty, reducing total and visceral fat, improving circulating lipid levels and lowering testosterone levels. There are not enough data to conclude whether insulin-sensitising agents improve hirsutism. The rationale for the use of metformin in infertility is that it lowers both circulating insulin levels and testosterone levels, and leads to increased ovulation.