Although the plasma insulin assay is now 40 years old, it is not widely used in clinical practice. However, simple methods (such as the various indexes relating fasting insulin to fasting glucose), the increase in plasma insulin at the 30th minute of an oral glucose tolerance test, and the increase in insulin or C-peptide after stimulation by glucagon are relatively reliable compared with more sophisticated approaches to assess beta-cell sensitivity to glucose, kinetics of insulin secretion, residual insulin secretion, or insulin sensitivity. But these measures are of no decisive help in distinguishing between the various forms of impaired fasting glucose or non-insulin-dependent diabetes, such as type 2 diabetes, slow type 1 diabetes, the various forms of maturity-onset diabetes of the young, or the mitochondrial genome defects. No data are available to show that the measurement of plasma insulin may be of help to adapt the treatment of a diabetic patient, except for the need of insulin therapy. There are some suggestions that fasting plasma insulin and, more precisely, the homeostasis model assessment indexes may help to predict the progression toward diabetes or the progressive deterioration of beta-cell function in diabetic patients.