Aerosol measurement in workplaces is carried out by industrial hygienists for a variety of different, but related reasons. The first is for the assessment of the workplace atmosphere so that aerosol concentrations there can be compared against the health-related exposure limits that are laid down, thus providing a basis for decisions on control action. The second is for the provision of valid measurements of worker exposure for the purposes of epidemiology. The third is for the measurement of aerosol concentrations in exhaust ventilation systems so that the effectiveness of emission control equipment can be assessed. In this chapter, the current state of aerosol science underpinning the first two of these is reviewed. This is followed by a discussion of some recent technical developments that have arisen, in particular in response to the recent emergence of the health-related particle size-selective criteria. Determination of aerosol exposure by sampling is not considered complete until the particles, which have been collected, have been quantified to provide an appropriate concentration.