A huge amount of different nanomaterials is nowadays on the market used for various specific applications. Some nanomaterials such as TiO2, ZnO as well as several other semiconductors exhibit photocatalytic activity. Hence these materials are used for many applications, e.g., for self-cleaning and antibacterial coatings on different surfaces and for the purification of wastewater where the cleaning can be induced by simple exposure to sunlight. Because of the frequent use of these nanoparticles it is important to investigate the life cycles of these nanostructured materials as well as their environmental impact and their toxicity to animals and humans. This review first gives a short overview about nanotechnology and nanotechnological products as well as about photocatalysis and semiconductors used in this field. We then discuss the need for a new technology named nanotoxicology and the problems occurring when investigating the toxic potential of nanomaterials as well as the life cycle of nanomaterials. Furthermore, we focus on the environmental impact of TiO2 and ZnO nanoparticles including toxic effects to bacteria, water organisms and plants as well as their toxic effects to humans including in vitro and in vivo studies.