A novel method for doping semiconductors is ion implantation where impurities are injected in the form of high-energy ions. The range of these ions is well defined and is a function of ion energy and mass, the nature of the crystal and its orientation to the beam. In addition t o its application to semiconductor device manufacture, ion implantation involves a good deal of interesting physics over a wide field, and in order to include the important aspects the article has been divided into two parts.