Aerosols, tiny airborne particles with diameters typically in the nano- and micrometer size range, are ubiquitous in the atmosphere and contribute to air pollution and negatively impact human health. They also impact atmospheric chemistry and biogeochemical cycles and play a key role in Earth’s climate through aerosol–radiation and aerosol–cloud interactions. For example, to form a cloud droplet, an aerosol particle is required. The process of cloud droplet formation on an aerosol particle occurs at the microscale but impacts macrophysical cloud properties and ultimately climate. In this chapter, we introduce key aerosol types often found in the atmosphere and discuss their physical and chemical properties. Aerosol particle size distributions, and the concepts of using logarithmic normal distributions to describe the distributions of aerosols in terms of their number, mass, volume, or surface area are discussed in detail. The chapter also discusses key aerosol sources and emissions, and introduces pathways and mechanisms how aerosols are formed. Aerosol removal processes, namely dry and wet scavenging processes, are discussed and the concepts of aerosol burden and lifetime are introduced.