Clouds can be considered as very complex reactors where multiphase chemistry takes place. Understanding these processes is crucial since they impact cloud formation and precipitation and thus the climate. Up to very recently only abiotic processes leading to the transformation of organic matter in clouds were studied by atmospheric scientists, they involved radical chemistry and particularly photochemistry. New actors have been now highlighted with the discovery of metabolically active microorganisms in cloud water. A new scientific question has been raised: are microorganisms able to biotransform organic compounds thanks to their metabolism" Are these biological processes competitive compared to abiotic processes" The aim of this chapter is to give updated information related to this emerging topic. The first part of this chapter introduces the cloud system, where a complex multiphase chemistry takes place leading to the transformation of chemical species in the gas, solid and aqueous phases. In this frame, during the cloud lifetime, cloud chemistry can lead to the formation of new low volatile compounds that will modify the physical and chemical properties of aerosols after cloud evaporation and can also contribute to the formation of secondary aerosols. The second part describes the cloud microbial population. About 105 bacteria and 103 fungi are present per mL of cloud water. The biodiversity is dominated by Sphingomonas, Pseudomonas, Dioszegia, Udeniomyces and Cryptococcus genera and shows similarities with the one found in dry atmosphere and precipitation. As a large fraction of the microbial cells is metabolically active, microorganisms could transform chemical species in atmospheric waters. The last part of this chapter focuses on the implication of microbial activity in cloud chemistry. Most of the studies concern the biodegradation pathways of mono- and dicarboxylic acids, methanol and formaldehyde. The quantitative comparison of microbial activity vs radical chemistry suggests that microorganisms could be active both during the day and during the night to biodegrade organic compounds in cloud water, while their activity will be only dominant at night in the case of methanol and formaldehyde. Moreover, very recent investigations are related to the biosynthesis of various compounds such as biosurfactants, and exopolymeric substances (EPSs) that could consequently modify the physico-chemical properties of aerosols. Although the results reported in the literature up to now are consistent with a potential contribution of microorganisms to atmospheric chemistry, some more work is needed to fully demonstrate and accurately quantify this contribution; new routes could be explored (macromolecules synthesis, oxidative stress metabolism).