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Microorganisms circulate in the atmosphere up to long distances, high altitudes and clouds. Since the early 2000's, we monitor and investigate microbial communities in cloud water at puy de Dôme Mountain summit in France (1465 m asl). We explore their biodiversity, their dynamics from aerosols to precipitation in relation with the meteorological and environmental contexts, and their interactions with atmospheric physico-chemical processes. Cultures and molecular approaches revealed a low biomass, but a high microbial richness composed of myriads of bacterial and fungal species from many origins and partly sustaining metabolic activity. These exhibit a functioning largely directed toward the fight against oxidants and osmotic variations, and the uptake of abundant small organic compounds, positioning clouds as stressful and probably selective microbial habitats. Microcosm incubation of natural cloud samples and numeric modeling indicated that biological activity can interfere with photochemistry in the transformation of organic compounds in clouds and thus contribute to their atmospheric and environmental fate. In the frame of the project NSERC “Frontières de la découverte”, bacteria antibiotic resistances are prospected within the huge diversity of the atmospheric microbiota.