Abstract: The atmosphere carries diverse and highly variable numbers of biological aerosol particles (i.e. bioaerosols) including microbial cells and their products and propagules. These are transported from sources to sinks with a residence time aloft of up to a few days, during which they can travel long distances and reach high altitudes. In the meantime, harsh atmospheric conditions impair cell’s activity, survival and integrity. Still, microbial cells have unique properties and some of them are capable of maintaining viability and metabolic activity in the atmosphere, notably in clouds, where their interactions with physical and chemical processes have possible impacts on hydrological and biogeochemical cycles. This tutorial will summarize the main peculiarities of microorganisms among aerosol particles, and provide an overview of some of the most promising methods for characterizing their biodiversity, viability, metabolic activity and functioning, and evaluate their environmental impacts. Major analytical challenges and requirements associated with the low microbial biomass and the high diversity and variability in the atmosphere will be discussed, along with difficulties and pitfalls that may arise when attempting to provide global estimates of their impacts.