The organization of photoresponsive molecular systems and nanomaterials on semiconductor surface holds great potential in the building of solar energy conversion devices where efficient energy conversion results from the optimized cooperation of several subsystems (semiconductor, dye sensitizers, redox mediator, hole transport medium), whose properties can be finely tuned through rational synthetic design. This chapter will review the fundamentals of semiconductor sensitization, a process relying on the quenching by charge transfer of molecular excited states coupled to semiconductor surfaces, and will move on by describing the structural and electronic properties of some of the most successful dye designs, used in conjunction with new electron transfer mediators in liquid electrolytes. From liquid electrolytes, a step forward is made by developing solid state hole conductors, which found their best employment in hybrid junctions with organo-halide lead perovskites, representing, at present, the most promising materials for solar-to-electric power conversion in mesoscopic solar cells. Finally, one of the most challenging tasks which can find solution by exploiting molecular level sensitized materials is discussed in detail through meaningful case studies: the production of solar fuels by photoelectrochemical water splitting.