Substances that are airborne and toxic by inhalation are usually evaluated by sampling and analysis of the relative or absolute concentration of the substance in air. First it is necessary to state the reason for evaluation: determining the sources of emissions in work processes; determining the quantity of emissions to the environment; determining compliance with regulations and guidelines; determining the exposure distribution for affected persons; determining the effect of process modification on exposure; and determining risk of disease. Once the desired outcome has been clearly stated it is possible to consider a strategy for sampling that involves sample number, distribution and timing. The sampling strategy also affects the selection of sampling and analysis methods, including requirements of selectivity, sensitivity and accuracy. Methods may be prescriptive, and following such methods strictly may be a requirement under regulation or for accreditation. Conversely, today's technology allows many options and variations that may not have been available in the past. Changing requirements, such as changes to concentration limit values, require a degree of flexibility. This chapter describes the theory and technology for sampling aerosols in general and beryllium aerosols in particular, with sufficient background and discussion to allow the user to select with confidence from the available options the tools and techniques most applicable to the desired outcome. It must be kept in mind that an air sample is only ever an estimate of exposure with an uncertainty that is associated with the methodology both of the strategy and the procedure.