This book reviews the modern state of the catalysts and photocatalysis of chemical reactions by semiconductor nanoparticles and nanoheterostructures with the main accent on the light-driven processes. A background and principles of the semiconductor nanophotocatalysis -- a new trend in photochemistry dealing with the photocatalytic redox-reactions with the participation of semiconductor nanoparticles -- is discussed. A general character of the fundamental principles of the nanophotocatalysts and the "classical" photocatalysis by microcrystalline semiconductors is combined with prominent differences originating from the spatial exciton confinement typical for ultra-small semiconductor nanoparticles.