This document provides a guideline for determining the number concentration and size distribution of ultrafine aerosols and nanoaerosols with mobility particle viewers (also known as differential mobility analyzers). In the case of ultrafine aerosols and nanoaerosols, exposure measures such as number and surface concentration are important. This document aids in assessing exposure to ultrafine aerosols and nanoaerosols in the workplace. In particular, the Differential Electric Mobility Spectrometer (DEMC) is discussed, which is now made available by several providers. It covers the principles of handling, problems with sampling in the workplace environment, calibration, maintenance of the equipment, measurement uncertainty and the reporting of measurement results.