Our current vision of habitable planets depends not only on the intrinsic properties of the planet such as bulk terrestrial composition, but on the characteristics of its host star. In general terms, high stellar energy radiation (X rays and extreme ultraviolet) can erode the planetary atmosphere, far and near ultraviolet drive the atmospheric chemistry, and visible and infrared fluxes control the planetary climate. Some crossovers exist, for example, the haze formation is produced by ultraviolet light and impacts the effect of the visible and infrared radiation on the planetary climate. This chapter outlines how stellar radiation influences the habitability potential of exoplanets.