Publisher Summary The ability of hydrogen (H) to affect dramatically the electronic properties of crystalline semiconductors, such as zinc oxide (ZnO), was recognized and was under systematic study by the early 1950s. Perhaps the earliest work on the behavior of hydrogen in elemental semiconductors is that of the diffusion of hydrogen through the walls of cylinders made of single-crystal silicon (Si) and germanium at high temperatures. In the mid-l970s, H chemisorption on Si surfaces was studied extensively. It was found that H can produce monohydrides and trihydrides on (111) Si surfaces and monohydrides and dihydrides on (100) Si surfaces. Furthermore, it was shown that hydrogenation leads to the reconstruction of surface atoms. In retrospect, it is clear that as the H–Si bond is stronger than the Si–Si bond, the surface dangling bonds can be readily terminated by H; therefore, strained Si–Si bonds can be relaxed by the insertion of H atoms, leading to the reconstruction of the Si surface. Also during the 1970s, research on the role of H in amorphous semiconductors intensified rapidly. The chapter highlights the current era of research on hydrogen in semiconductors.