This European Standard gives guidelines on the measurement of the particle fraction of the aerosol. The gas phase will not be considered. For ultrafine aerosols and nanoaerosols, exposure metrics such as the number and surface area concentration could be important. This European Standard also gives guidelines for the determination of workplace exposure to ultrafine aerosols and nanoaerosols by measuring the particle number concentration with a condensation particle counter (CPC). However, a CPC measures up to several µm and does not discriminate between particles of different sizes or origin. Specifically, the CPCs, using different working fluids and technologies, are discussed. Principles of operation, problems of sampling in the workplace environment, aspects for selecting a suitable instrument, limits of application, calibration, equipment maintenance, measurement uncertainty, and reporting of measurement results are covered. Potential problems and limitations are described, which need to be addressed if workplace measurements are carried out.