Monoclonal antibodies (MAb) are antibodies that are identical because they are produced by one type of immune cell; all are clones of a single parent cell. Given (almost) any substance, it is possible to create monoclonal antibodies that specifically bind to that substance; they can then serve to detect or purify that substance. Monoclonal antibodies (MAbs) are an integral part of targeted therapy approach for various diseases which result in decrease in adverse effects and increase in efficacy. They target various receptors or various growth factors on the cell surface and modulate their vital functions and cause cell death by various mechanisms. They are produced by conventional hybridoma technique. They have been modified to humanized forms to decrease adverse effects. Since then many MAbs have been produced which have changed the clinical course of many diseases. In this article, briefly the technique of production of these monoclonal antibodies, their mechanism of action in therapy and clinically important monoclonal antibodies will be discussed.