Metformin is an old insulin sensitizer that has been widely used in women with polycystic ovary syndrome (PCOS) to treat metabolic comorbidities and may also improve ovarian dysfunction in women with PCOS. In fact, metformin may improve insulin resistance, a common finding of PCOS, and reduce insulin blood levels. In this way, androgen production rates can be reduced, and in a subset of women, menses abnormalities and ovulatory rates may improve. The current Endocrine Society Guidelines recommend the use of metformin during adolescence, particularly when excess body weight is present. In the presence of obesity and glucose intolerance states, particularly if those patients fail to modify their lifestyles, metformin may have some significant benefits. Finally, although this drug should not be used as a first-line treatment for ovulatory dysfunctions in adult women with PCOS, there are data supporting the suggestion that in many cases pretreatment with metformin may favor ovulatory response to clomiphene citrate. This article summarizes the available evidence in the favor of metformin use in women with PCOS and emphasizes the need for an individualized therapeutic approach.