M dwarfs are prime targets of current and upcoming exoplanet searches, because they comprise 75% of the sun’s neighbors, they have ~2.5 small planets per star, and their planets are comparatively easy to detect. Recently, a potentially habitable planet was discovered around our nearest stellar neighbor, Proxima Cen, an M dwarf. M dwarf terrestrial planets, like Proxima Cen b, will likely be the first targets of upcoming atmospheric characterization efforts. Critical to these efforts will be the knowledge of the stellar spectral energy distribution (SED), especially at high energies, and how the SED changes over short and long timescales. UV and X-ray stellar radiation impacts planetary atmospheres through heating and photochemistry, even regulating production of potential biomarkers. Together with stellar energetic particles, the star can drive atmospheric loss. In this talk, I will review what we know today about M dwarfs’ high-energy radiation, largely from the MUSCLES Treasury Survey, and how we can infer M dwarf UV radiation from optical chromospheric tracers after Hubble. I will also discuss current and future tools we have of estimating stellar energetic particles (including CMEs), and the importance of including particles when modeling the effects of large stellar flares on an exoplanet’s atmosphere.