This monograph provides a comprehensive treatment of the properties of hydrogen as an impurity, beginning at an introductory level. It describes the states and binding sites that hydrogen assumes in the lattice, the passivation of shallow and deep impurities, the properties of hydrogen-related defects in semiconductors, the diffusivity of the various forms of hydrogen, and the effects of hydrogen on the mechanical properties of semiconductors. Emphasis is placed on the ways in which hydrogen is incorporated during crystal growth and device fabrication, an area of tremendous practical importance.