The fine aerosol fractions of interest in occupational health are those associated with the regional deposition of inhaled aerosols in the different parts of the human respiratory tract. The recent recommendations of the International Standards Organization, the American Conference of Governmental Industrial Hygienists and the Comité Europeén de Normalisation define the thoracic fraction (inhaled particles which penetrate into the lung) and the respirable fraction (inhaled particles which penetrate further into the alveolar region). Based on the results of inhalation experiments with humans, conventional curves for these fractions are proposed, which provide 'yardsticks' for health-related sampling devices. Samplers for the respirable fraction have been in existence since the early 1950s, and a fairly large range of acceptable instrumentation is available commercially. Recently, however, attention has been drawn to the effect of even small departures from the 'target' curve on the respirable mass sampled. This suggests the need for additional criteria for sampler acceptability, based on sampled mass in addition to particle size selectivity. Samplers for the thoracic fraction may be based on similar physical concepts to those for respirable aerosol. In addition, versatile devices capable of providing information about more than one fraction simultaneously are being proposed and are finding increasing use as investigative tools for industrial hygienists. Finally, measurement of fine fibrous aerosols is recognized as a special case, and so is treated separately.