Polycystic ovary syndrome (PCOS) and Type 2 diabetes mellitus (T2D) are both obesity-related conditions that share epidemiological and pathophysiological factors. Insulin resistance is a key factor whereby obesity influences the expression of each condition. However, the mechanisms by which insulin resistance contributes towards the manifestation of PCOS and T2D differ in important ways: in PCOS, compensatory hyperinsulinemia results in pleiotropic effects including co-gonadotrophic stimulation of ovarian and adrenal steroidogenesis; in T2D, insulin resistance contributes towards β-cell exhaustion and ultimately to hyposecretion of insulin with resultant dysglycemia. The link between PCOS and Type 1 diabetes mellitus is believed to implicate supraphysiological concentrations of insulin within the systemic circulation. Further progression of the obesity epidemic will ensure even greater prominence of important obesity-related conditions such as PCOS and T2D. Research to gain a clearer understanding of the mechanisms linking each condition should be a priority.