Air pollution has been recently classified among the top ten risk factors for mortality worldwide. The evidence on the long-term effects of air pollutants is mounting, mostly from multi-centre American studies or longitudinal studies conducted in single European cohorts. Recently, the EU-funded project ESCAPE (European Study of Cohorts for Air Pollution Effects) involved more than 30 cohort studies with the aim of producing pooled estimates of the long-term health effects of ambient air pollution at European level. The project developed a standardized and flexible methodology to estimate chronic exposure to several air pollutants, applied such estimates to existing cohorts in Europe, and analyzed the exposure-response relationships with different health endpoints, including adverse pregnancy outcomes, respiratory diseases among children, cardio-respiratory diseases among adults, cause-specific mortality and lung cancer incidence. One of the most important results has been the detection of relevant health effects of particulate matter at concentrations below the current air quality limit values in Europe.