The question of ​whether or not we are alone in the universe has fascinated humanity for many centuries. Upcoming missions such as the James Webb Space Telescope (JWST), the Atmospheric Remote-sensing Infrared Exoplanet Large-survey (ARIEL), or the European Extremely Large Telescope (E-ELT), will deliver data addressing this crucial question within the next few decades. Since the dawn of the exoplanet era, some twenty-five years ago, surveys based on the transit – and radial – velocity methods have confirmed the existence of several thousand exoplanets, of which currently 49 (according to ​Planetary Habitability Laboratory​) lie within the habitable zone (HZ) (see, e.g., ​Kane et al., 2016​, Morton et al., 2016​, respectively). Of particular interest are exoplanets with known masses and radii consistent with rocky interiors. Small orbital separations for habitable planets orbiting K- and M-dwarf stars increase detection probability due to, e.g., favorable (planet/star) flux ratios. Detections of numerous rocky exoplanets in the HZ of G, K, and M-dwarf stars are expected in the near future by both ground-based detection surveys (e.g., MEarth) and the upcoming space missions CHEOPS (ESA) and PLATO2.0 (ESA, see, e.g., ​Rauer et al., 2016​). However, recent estimations show that the exoplanetary radiation environment around certain K- and M-dwarf stars may be much harsher than what we experience from the Sun (see, e.g., Herbst et al., 2019a​). Additionally, due to small planet-star separations, an Earth-like exoplanet could be exposed to an enhanced stellar radiation environment, which – in turn – would affect its habitability, e.g., due to a hazardous flux of stellar energetic particles (SEPs) which influence its atmospheric evolution, climate, photochemistry (see, e.g., ​Grenfell et al., 2012​; ​Scheucher et al., 2018​) as well as the altitude-dependent atmospheric radiation dose (​Atri, 2017​). The influence of higher energy particles upon non-Earth-like atmospheres dominated by CO2, H2, and H2O and the influence on biosignatures and climate is a newly emerging topic in exoplanet science. Thereby, detailed knowledge of the impact of the stellar radiation and particle environment on the (exo)planetary atmospheric chemistry, climate, and induced atmospheric particle radiation field is crucial to assess its habitability and, in particular, potential atmospheric biosignatures.