One of the key reasons why semiconductors are technologically so useful is that their electrical, and to a certain extent their optical properties can be dramatically modified by the addition of trace concentrations of impurities. This is true for a wide range of elemental and compound semiconductors. Although the summary which follows specifically addresses silicon, the general thrust of the arguments may be applied to other group IV, III-V and II-VI materials with due regard for the valency of the host atoms and impurities.