Publisher Summary Insulin exerts a wide spectrum of effects at the cellular level. These include effects at the membrane level, for example, the stimulation of glucose and amino acid uptake; effects on both membrane and cytoplasmic enzymes; and effects on protein synthesis, DNA synthesis, cell growth, and differentiation. This is the reason why unraveling the mechanism of insulin action has been difficult. This chapter describes insulin receptors, receptor antibodies, and the mechanism of insulin action. In addition, the effects of insulin vary both in time course and dose–response. Some effects of insulin, such as the stimulation of glucose transport, occur within seconds after hormone exposure, whereas others, such as the stimulation of DNA synthesis, require hours; effects on most cytoplasmic enzymes lie somewhere between these two extremes. It is well known that insulin stimulates glucose transport, and this effect persists in membrane vesicles even after the cell is broken. Several new probes have become available that have shed some light on the problem of insulin action. These include cultured cell systems, which are highly responsive to insulin; new methods for studying receptor structure; mutant cell lines that possess defects in either the insulin receptor or pathways of insulin action; and antibodies to the insulin receptor that block insulin binding and mimic insulin action.