This chapter introduces the reader to one of the defining attributes of semiconductors: the ability of adjusting the concentration of mobile charge carriers by doping. Following a brief examination of the electron configuration of atoms and of bonding—particularly for the technologically important cases of group IV elemental semiconductors and III–V compound semiconductors—the concept of substitutional doping and the hydrogen model for the energy-level structure of dopants are developed. Examples on the well-established substitutional doping of diamond cubic and zinc-blende semiconductors are complemented by an analysis of “self-doping” by point defects and unintentionally introduced impurities, which often hinders the broad application of many emerging semiconductor materials and remains the subject of active research.