During the past decade, the potential implications of insulin resistance were recognised by clinicians ranging from endocrinologists to cardiologists. Central to this expanding interest is Reaven's hypothesis that tissue resistance to the effects of insulin is a factor linking various metabolic disorders and coronary heart disease. This review critically describes the different approaches for the evaluation of insulin sensitivity in vivo. Qualities and limitations of several investigative techniques are discussed, such as anthropometric indexes, basal biological indexes, insulin suppression tests and insulin tolerance tests. The two most widely used methods for quantifying insulin sensitivity are the euglycaemic hyperinsulinaemic clamp and the intravenous glucose tolerance test with minimal model analysis. Insulin resistance occurs in many aetiologically diverse human disorders. Genetic syndromes with extreme insulin resistance are very uncommon. Insulin resistance is frequently associated with obesity, type 2 diabetes and essential hypertension. The insulin resistance syndrome called syndrome X includes impaired insulin-mediated glucose uptake, impaired glucose tolerance, hyperinsulinaemia, hypertension, dyslipidaemia and haemostatic disorders. Finally, the clinical significance of high values of insulin sensitivity is discussed.