TiO(2) is one of the most investigated compounds in contemporary materials science. Due to a set of virtually unique electronic properties, it finds intense use in photoelectrochemical applications such as photocatalysis or solar cells. The main drawback in view of direct exploitation of solar-light-based effects is its large band gap of >3 eV. Visible-light-activated TiO(2) can be prepared by doping (band-gap engineering) through incorporation or decoration with other metal ions, nonmetal ions, and semiconductors. Most recently, efforts in TiO(2) research have been even more intensified by the finding of self-organized nanotubular oxide architectures that can be prepared by a simple but optimized anodization of Ti metal surfaces. These nanotubular geometries provide large potential for enhanced and novel functional features. This Review examines doped TiO(2) and in particular TiO(2) nanotubes. Various types of dopants, doping methods, and applications of modified TiO(2) nanotubes are discussed.