Adverse health effects of exposure to particles have been described in numerous epidemiological studies. Health endpoints thoroughly studied are all cause and cause-specific mortality, and hospital admissions. Older studies focussed on associations with PM10 (then named fine particles). During the last decade, PM2.5 was increasingly emphasised, and the term "fine particles" was restricted to this size fraction. Currently, ultrafine particles (UF, PM0.1) are discussed to be another important fraction which should be characterised by particle number instead of particle mass. However, data on UF exposure and health effects are still limited. The mechanisms by which particles influence human health are only poorly understood. Under discussion is the role of particle size and particle composition. The risk assessment of coarse particles (i.e. the size fraction between 2.5 and 10 microm) suffers from inconsistent findings. The question of causality is not completely answered. However, it is widely accepted that PM is some kind of container including components which are toxicologically relevant and others which might be seen mainly as indicators. Thus, the local mix may influence the toxicological potency of PM, and results from studies carried out in one region may not necessarily be consistent with results gained elsewhere. Recently, reanalyses of epidemiological studies performed by the Health Effects Institute (HEI) qualitatively confirmed the original results. New insight in the influence of socioeconomic factors extended the knowledge on health effects of particles. To some extent, the slope of the dose response relationships from time-series analyses needed downward adjustment due to some problems with statistical analysis programmes. Nevertheless, the whole body of knowledge supports the role of PM as a type of air pollution with great influence on human health.