The habitability of exoplanets likely depends on the presence of such molecules as oxygen, ozone, water, methane, and carbon dioxide in their atmospheres. Photoionization of these molecules is driven by the far-ultraviolet radiation from host stars. The hydrogen Lyman-alpha line dominates the far-ultraviolet spectra of solar-like and cooler dwarf stars, but only a small fraction of the instrinsic stellar Lyman-alpha radiation is observable from Earth because of interstellar absorption. I will present several techniques for reconstructing the intrinsic Lyman-alpha radiation of dwarf stars with spectral types F-M. The extreme-ultraviolet radiation from host stars photoionizes hydrogen in the atmospheres of exoplanets, heats the outer atmospheres, and drives mass loss. Since most of the stellar EUV emission is absorbed by interstellar gas, I will describe a new approach for estimating the EUV emission of solar-type and cooler stars.