ISO 28439:2011 provides guidelines for the determination of the number concentration and size distribution of ultrafine aerosols and nanoaerosols by use of mobility particle sizers (also called differential mobility analysers). Only the particle fraction of the aerosol is considered. For ultrafine aerosols and nanoaerosols, exposure metrics such as the number and surface area concentration are important. ISO 28439:2011 also gives guidelines for the determination of workplace exposure to ultrafine aerosols and nanoaerosols. Specifically, the differential mobility analysing system (DMAS), now available from several vendors, is discussed. Principles of operation, problems of sampling in the workplace environment, calibration, equipment maintenance, measurement uncertainty, and reporting of measurement results are covered. Potential problems and limitations are described, which need to be addressed when limit values are fixed and compliance measurements carried out.