Heterojunction semiconductors are discussed as a strategy to develop efficient visible light photocatalysts for water splitting. The concept has been demonstrated in photovoltaic cells and optoelectronic devices, for which junction-type semiconductors show greatly enhanced efficiency compared to the devices consisting of a single semiconductor. We applied this proven concept to fabricate photocatalysts of inorganic semiconductors. Thus heterojunction structures of Shottky junctions, p-n junctions (or p-n diode), p-n junctions with Ohmic layer, and bulk heterojunctions were fabricated and their photoactivity was tested for reduction or oxidation of water under visible light. The formation of heterojunctions results in the efficient separation of electron-hole pairs to minimize the energy-wasteful electron-hole recombination, which leads to the high photocatalytic activity. As the complexity and sophistication of the photocatalyst fabrication increased, the photoactivity also increased. Modern nanomaterial synthetic techniques were employed to bring into reality the highly engineered material configurations. (C) 2011 Elsevier B. V. All rights reserved.