Aerosols are defined as any mixture of solid or liquid particles/droplets that are stable as a suspension in air. Aerosols influence the lives of a large majority of the population. A proportion of particulates exacerbate or induce lung disease. Other aerosols are used in the prevention or control of lung disease or in the investigation of disease. This review covers the mechanisms of deposition of aerosols, the production and sizing of aerosols, factors affecting the variability in output from nebulisers and the use of radionuclides in defining drug deposition from therapeutic nebulisers. © 1993 Springer-Verlag.