Numerous epidemiological studies have been carried out during the last decades which have demonstrated an association between the pollution of outside air with toxic substances and the occurrence of health-related effects. Against the background of these findings, particularly in recent years, the focus of research has clearly shifted towards particulate matter (PM), notable fine and ultrafine particles. While diverse measurements of PM in the outside air have been conducted, only few data on indoor air pollution are available. The concentration of PM in the indoor environment is highly variable in time and space due to various influencing factors like type of the source, building and room characteristics, the activities of users and the airing behaviour. In this article we aim to summarise and discuss the exposure situation regarding PM in indoor environments. In residences, European studies have found mean PM (2.5) values between 10 and 87 microg/m (3). Especially in smokers' homes, a high background level was observed, reaching very high concentrations of some hundred microg/m (3) when active smoking took place. There are some studies on air quality in schools and similar public places which show that exposure to particulate matter in these environments is high. The main causes of this situation appear to be an insufficient ventilation routine as well as the low frequency and quality of cleaning. In combination with the high number of pupils in relation to room area and volume and their sometimes high physical activity, this leads to a continued resuspension of particles from the room's surfaces. A very high concentration of PM can be observed in those recreational places where smoking is not prohibited, such as discotheques, pubs and restaurants. Here, the mean PM values can reach some hundred microg/m (3). Specific strategies are necessary to especially protect the health of non-smokers in such places. Further investigations are needed to characterise the composition of indoor particles and their toxicological properties compared to particles from outdoor origin.