Metal nanoparticles (MNPs) are widely used in photocatalysis as plasmonic photocatalyst or as cocatalyst supported on semiconductor photocatalysts. They play a crucial role in the adsorption and the activation processes of the reactants and the intermediates as well as in the separation of the generated electrons/holes charges. In this chapter we highlight the utility of the MNPs in various photocatalytic processes. First, we report and discuss the optical and electronic behaviors of the MNPs and how these properties make them promising in photocatalysis. Then, we discuss their different roles in photocatalytic, water splitting, CO2 reduction and green chemistry.