This review discusses the utilization of photocatalysis for the degradation of water pollutants. Emphasis is placed on TiO2 nanoparticles as a benchmark photocatalyst for the destruction of microorganisms and the degradation of a wide variety of organic compounds like phenolics, dyes, pesticides and pharmaceuticals. The mechanism of photocatalytic degradation is elucidated, underlining the importance of reaction kinetics for the efficient design of the processes. The effects of different reaction parameters on photocatalytic degradation are discussed. Surface modification of TiO2 for visible light response by doping and heterostructuring is outlined. Finally, the challenges in the implementation of this technology for "real" waste water systems are summarized with a few attainable goals.