Insulin is one principal hormone in mammals. It modulates the process of metabolism and promotes cellular growth and differentiation. Its action is mediated through insulin receptor. Insulin binding activates the tyrosine kinase activity of the receptor. The activated receptor frequently undergoes autophosphorylation and then phosphorylates cellular signal molecules, thus making it possible for insulin to elicit the corresponding response. A brief description is given on the process of signal transduction in the cell after insulin stimulation and the cellular signal molecules involved in the process.