This document specifies the general principles, including testing procedures and requirements, for the classification of performance of low-cost sensor systems for the monitoring of particulate matter in ambient air at fixed sites. The classification of sensor systems includes tests that are performed under prescribed conditions. It does not guarantee performance in locations that are different from the tests, variations in meteorological climate from the test programme or account for stability over time, which can only be assessed under ongoing quality control strategies. The described procedure is applicable to the determination of the mass concentration of particulate matter. The pollutants that are considered in this document are PM10 and PM2,5 in the range of concentrations expected in ambient air. This document provides a classification that is consistent with the requirements for indicative measurements and objective estimation defined in Directive 2008/50/EC. In addition, it provides a classification for applications (non-regulatory measurements) that require more relaxed performance criteria. This document applies to sensor systems used as individual systems. It does not apply to sensor systems as part of a sensor network. However, for some applications (e.g. in cities) sensor systems are deployed as part of a sensor network. Annex A provides information on the use of sensor systems as nodes in a sensor network.