Ozone is a highly oxidative compound formed in the lower atmosphere (from gases originating to a large extent from anthropogenic sources) by photochemistry driven by solar radiation. Owing to its highly reactive chemical properties, ozone is harmful to vegetation, materials and human health. In the troposphere, ozone is also an efficient greenhouse gas. This report summarizes the results of a multidisciplinary analysis to assess the effects of ozone on health. The analysis indicates that ozone pollution affects the health of most of the populations of the WHO European Region, leading to a wide range of health problems. The effects include some 21,000 premature deaths each year in 25 countries in the European Union on and after days with high ozone levels. Current policies are not sufficient to reduce ozone levels in the Region or their impact in the next decade.