Bacteria are present everywhere around us; in water, soil, food, on our skin, and in the air we breathe, mainly as parts of aerosols (i.e. minute air-suspended particles). Aerosols are mainly composed of sea salt, dust, sulphates and soot but some also contain bacteria, virus, pollen, plant debris and organic matter (i.e. bioaerosols). Aerosols are of central importance in processes that affect Earth’s climate e.g. by controlling scattering of solar radiation, cloud formation and precipitation. Clouds cover approximately 50-60% of Earth’s surface and act as the main cooling factor. For clouds to form, an ice nuclei or cloud condensation nuclei is needed such as sea salt, soot and dust, but also biological material such as bacteria and phytoplankton. Due to the difficulties related to air sampling, few studies have investigated in detail the significance of bioaerosols on atmospheric processes. However, with the development of new molecular techniques the possibilities to study the atmosphere from a biological perspective have dramatically increased.This thesis work focuses on four central issues for understanding the role of bacteria in the atmosphere: 1) how different sampling techniques may affect studies of the bacterial community, 2) the composition of the bacterial community and its origin, 3) sources, transport and deposition of airborne bacteria and, 4) ejection of bacteria into the atmosphere from the ocean. The studies show that the airborne bacterial community is highly diverse and that its origin can be traced back to both marine and terrestrial environments. By use of simulation models, we have followed sampled air parcels from their source to the sampling point. In a study conducted in Kalmar we showed that the major source of bacteria in the atmosphere was long-range transport from marine waters. We have throughout the studies detected several pathogenic bacteria along with isolates important for atmospheric processes. Thus, several bacteria known to act as ice and cloud condensation nuclei were detected, such as Pseudomonas fluorescence, Xanthomonassp., Pantoea agglomerans and the diatom Thalassiosira pseudonana. After characterization of the atmospheric bacterial community and its sources, we investigated the emission of bacteria from seawater to aerosols. We show that a wide diversity of bacteria and phytoplankton can be emitted from seawater to the atmosphere, and that the transfer efficiency for specific species vary significantly. Considering that microorganisms differ in key cell surface characteristics, ourfindings suggest that determining the identity and diversity of microorganisms in bioaerosols is critical for deducing their role in atmospheric processes.