1. Insulin resistance is an early and major feature in the development of non-insulin-dependent diabetes mellitus (NIDDM), but it is also associated with hyperlipidaemia, hypertension, obesity and cardiovascular disease, the so-called 'insulin-resistance syndrome' (Syndrome X). 2. There is a strong genetic determination of NIDDM and insulin resistance, but the environmental factors of calorie excess, reduced activity and obesity also make a major contribution. 3. Central (abdominal) obesity is much more strongly associated with insulin resistance than is overall obesity. From twin studies, there appears to be specific genetic determinants of central abdominal fat, independent of overall obesity. 4. Calorie restriction and weight loss improve insulin sensitivity in overweight humans. Isocaloric alteration of macronutrients substantially affects insulin sensitivity in rats but not, at least in the short-term, in humans. 5. Exercise training improves insulin sensitivity via increased oxidative enzymes, glucose transporters (GLUT4) and capillarity in muscle as well as by reducing abdominal fat. 6. Metformin has been the only available drug that has been used clinically to significantly improve insulin sensitivity, but the new 'glitazones' (thiazolidinediones) have a more specific effect via altered lipid metabolism.