Insulin generates a complex assay of biological responses in a variety of cell systems. The first step in insulin action is binding of the hormone to its specific cell surface receptor. This receptor is an oligomer consisting of two α-subunits with Mr 130 kDa, and twos-subunits with Mr 95 kDa; the different subunits are linked together by disulfide bridges. The α-subunit contains most — if not all — of the hormone binding domain, whereas the s- subunit is an insulin-sensitive tyrosine kinase. A general consensus has been reached over the last years indicating that the insulin receptor tyrosine kinase is necessary for mediating insulin action. In the present review we will briefly summarize the insulin receptor kinase characteristics, and the evidence showing the crucial role of the receptor kinase in generation of insulin’s effects.