The study of hydrogen in solids is of significant importance in modern technology. Often present in large concentrations by various growth techniques, hydrogen affects the structural, electronic, and optical properties of crystals. Isolated hydrogen is often electrically active, causing passivation or charge compensation of defects and impurities. Since the presidential proposal of the Freedom Car, hydrogen in semiconductors takes another twist as a potential source of clean fuel for the future. The study of hydrogen transport, diffusion, and chemical reaction in solids is, therefore, of significant social importance. But as often happens in science, different groups of researchers studying hydrogen in solids do not always speak the same language. For example, few people would consider silicon as a practical fuel storage material due to its weight, but in reality, aluminum hydrides have been seriously considered for such a purpose. The knowledge acquired in the last half century for hydrogen in conventional semiconductors has not been effectively transferred to the study of hydrogen storage in emerging semiconductors such as carbon nanotubes. This volume brings together scientists from various research areas to encourage cross fertilization and wider dissemination of the advances in this important field of study. Highlights include recent developments in the understanding of light-induced metastability in amorphous silicon, hydrogen in zinc oxide as a means of n-type doping, and hydrogen doping of dilute nitrides that modifies the bandgap in some rather surprising ways. Hydrogen interaction with donors and acceptors is a never-ending story that is also featured here. Topics include: hydrogen in elemental semiconductors; hydrogen in oxides; and general properties of hydrogen in semiconductors.