Insulin resistance may be said to exist whenever normal concentrations of insulin produce a less than normal biologic response. Hormone resistant states may be divided into those due to decreased sensitivity to a hormone (i.e., a shift in the dose-response curve to the right), those due to a decrease in the maximal response to the hormone, and those that are combinations of decreased sensitivity and decreased responsiveness. This distinction is important, since the molecular mechanisms that produce these various forms of insulin resistance may be different. Disorders associated with alterations prior to the interaction of insulin with its receptor are more likely to produce states of decreased sensitivity, where disorders associated with alterations at the intracellular steps in insulin action are more likely to produce decreased responsiveness. Alterations in the insulin receptor itself may produce either, although most frequently changes in receptor affinity as well as in receptor number will be manifest as changes in sensitivity. This is a result of the large number of “spare” receptors for most insulin effects. In many studies, the differential diagnosis between states of altered sensitivity and altered responsiveness is difficult due to the complicated and interrelated nature of the metabolic pathways of insulin action. This is frequently further complicated by incomplete data (usually the result of studying response to only one hormone concentration rather than a full dose-response) and change in rates of basal metabolism in different diseases. The latter is a particularly difficult problem, but it is clear that use of “fold-” or “percent-” stimulation may further obscure the nature of the change when complete! data are not provided. With more precise use of these terms and a more complete understanding of insulin action, it will be possible to begin to segregate the roles of the various prereceptor, receptor and postreceptor factors that are involved in producing the differing patterns of metabolism observed in disease.