Lecture in the series of the ECSA Air Quality Working Group monthly meeting on 7 May 2021. The lecture is about citizen science with common PM2.5 sensors Nova SDS011 and Sensirion SPS30 by Joost Wesseling of Samen Meten (RIVM Rijksinstituut voor Volksgezondheid en Milieu in the Netherlands. The two sensors mentioned are the most commonly used ones in Europe, in particular the SDS011 is much used. Joost explains how they work, how they perform, what you can do with the data, how they compare to official data, the corrections and calibrations you can do.