This chapter discusses the mechanisms of insulin action. It has been firmly established that in many mammalian cell systems, insulin, when added to intact cells, increases the transport of specific monosaccharides, amino acids, and electrolytes. Insulin binds to cells in which it enhances transport or performs another type of activation and does not bind to insensitive cells. The hormone binds specifically to a protein in the cell membrane, and this protein is the so-called insulin receptor molecule. The receptor has been purified and its properties are being studied by many ingenious techniques. The perturbation of the membrane and the resulting transport effects caused by insulin can be satisfactorily imitated by anoxia—certain inhibitors of oxidative phosphorylation, phospholipases, and proteases. These observations suggest that the hormone-receptor interaction is associated with some breakdown process of membrane constituents, perhaps within a specific lipoprotein complex. The nontransport effects of insulin do not seem to regulate the membrane effects. It has been shown that the inhibition of protein synthesis by puromycin does not affect the enhancement of glucose transport. Insulin affects transport in cell membranes in the virtual absence of cytoplasm and cell organelles. It may well be that the membrane effects of insulin trigger the production of a specific second messenger, which activates a reaction shared at some point by the processes of anabolism—the production of glycogen, protein, fat, and nucleic acid from the smaller precursors.