Semiconductor-based photocatalysis has attracted much attention in recent years because of its potential for solving energy and environmental problems that we are now facing. Among many photocatalytic reactions, the splitting of H2O into H2 and O2 and the reduction of CO2 with H2O into organic compounds such as CH4 and CH3OH are two of the most important and challenging reactions. Many studies have been devoted to designing and preparing novel photocatalytic materials for these two reactions. This article highlights recent advances in developing semiconductor-based nanocomposite photocatalysts for the production of H2 and the reduction of CO2. The systems of semiconductor–cocatalyst, semiconductor–carbon (carbon nanotube or graphene) and semiconductor–semiconductor nanocomposites have mainly been described. It has been demonstrated that the design and preparation of nanocomposites with proper structures can facilitate charge separation/migration and decrease the charge recombination probability, thus promoting the photocatalytic activity. Keeping the reduction and oxidation processes in different regions in the nanocomposite may also enhance the photocatalytic efficiency and stability. The location and size of cocatalysts, the interfacial contact between semiconductor and carbon materials, and the heterojunctions between different semiconductors together with the suitable alignment of band edges of semiconductors are key factors determining the photocatalytic behaviours of the nanocomposite catalysts.