Two-dimensional transition metal dichalcogenides (2-D TMDCs) have unique band structures and show much promise for photochemical and photovoltaic applications. This review focuses on the latest 2-D TMDC researches relating to their use in next-generation ultrathin solar cell and photocatalytic systems. The fundamental properties that give rise to the high optical conductivity of 2-D TMDCs are discussed. The short optical absorption path in 2-D systems limits their use in light-harvesting applications, and various strategies to address this limitation are considered. For example, 2-D TMDCs can be stacked to form an energy-cascade-type solar cell with broadband absorption across the solar spectrum. By combining various heterostructures, the energy offset at the interface can be tuned to achieve efficient separation of photoexcited electron-hole pairs and create a direct band gap. Solution-processed chemistry can be used to synthesise foam-like, porous, interconnected TMDC networks which can provide efficient light harvesting and energy conversion. (C) 2015 Elsevier Ltd. All rights reserved.