Oxide semiconductors exhibit a range of physical properties and have potential optical, electronic, and energy applications. Transparent conducting oxides (TCOs) are currently used in products such as flat-panel displays. The prevailing n-type conductivity in these materials has historically been attributed to native defects such as oxygen vacancies. Recent calculations and experiments, however, have provided evidence that native defects are actually not responsible in majority of the cases. Hydrogen, on the other hand, does act as a shallow donor and can dramatically affect the electrical properties of oxides. In addition to contributing to n-type doping, hydrogen also passivates dangling bonds in cation vacancies and passivates acceptor dopants. Some oxides contain “hidden hydrogen,” perhaps H2 molecules, which dissociate at elevated temperatures. In this article, the many roles of hydrogen in zinc oxide, tin dioxide, titanium dioxide, indium (III) oxide, gallium (III) oxide, and strontium titanate are reviewed. The emphasis is on fundamental electronic, structural, and vibrational properties of hydrogen complexes, as determined by experiment and theory.