Aeroplankton (or aerial plankton) are tiny lifeforms that float and drift in the air, carried by wind. Most of the living things that make up aeroplankton are very small to microscopic in size, and many can be difficult to identify because of their tiny size. Scientists collect them for study in traps and sweep nets from aircraft, kites or balloons. Aeroplankton is made up mostly of microorganisms, including viruses, about 1,000 different species of bacteria, around 40,000 varieties of fungi, and hundreds of species of protists, algae, mosses, and liverworts that live some part of their life cycle as aeroplankton, often as spores, pollen, and wind-scattered seeds. Additionally, microorganisms are swept into the air from terrestrial dust storms, and an even larger amount of airborne marine microorganisms are propelled high into the atmosphere in sea spray. Aeroplankton deposits hundreds of millions of airborne viruses and tens of millions of bacteria every day on every square meter around the planet. Small, drifting aeroplankton are found everywhere in the atmosphere, reaching concentration up to 106 microbial cells per cubic metre. Processes such as aerosolisation and wind transport determine how the microorganisms are distributed in the atmosphere. Air mass circulation globally disperses vast numbers of the floating aerial organisms, which travel across and between continents, creating biogeographic patterns by surviving and settling in remote environments. As well as the colonization of pristine environments, the globetrotting behaviour of these organisms has human health consequences. Airborne microorganisms are also involved in cloud formation and precipitation, and play important roles in the formation of the phyllosphere, a vast terrestrial habitat involved in nutrient cycling. Overview The atmosphere is the least understood biome on Earth despite its critical role as a microbial transport medium. Recent studies have shown microorganisms are ubiquitous in the atmosphere and reach concentration up to 106 microbial cells per cubic metre () and that they might be metabolically active. Different processes, such as aerosolisation, might be important in selecting which microorganisms exist in the atmosphere. Another process, microbial transport in the atmosphere, is critical for understanding the role microorganisms play in meteorology, atmospheric chemistry and public health. Changes in species geographic distributions can have strong ecological and socioeconomic consequences. In the case of microorganisms, air mass circulation disperses vast amounts of individuals and interconnects remote environments. Airborne microorganisms can travel between continents, survive and settle on remote environments, which creates biogeographic patterns. The