Fine particulate matter is a heterogeneous and complex mixture that varies over time and space. It can have diverse physical characteristics and consist of a multitude of chemical components, many of which have been described as potentially toxic. Consequently, identifying and quantifying the influences of specific components or mixtures associated with specific sources on health effects is one of the most complex areas of environmental health research, particularly when particles interact with other co-pollutants. Current knowledge does not allow for precise quantification or ranking of the compounds posing the greatest risk, and the observed health associations may result from multiple components acting on different physiological mechanisms. Some findings suggest a degree of differential toxicity, i.e., more frequent associations between traffic-related particulate matter, fine and ultrafine particles, particulate metals, and elemental carbon, and various serious health effects, such as increased mortality and morbidity linked to cardiovascular and respiratory diseases. A carefully targeted program of toxicological and epidemiological research, incorporating more sophisticated approaches (such as the collection of more speciation data, more refined modeling techniques, precise exposure assessment, and a better definition of individual susceptibility) and optimal collaboration among multidisciplinary teams, is now necessary. This will facilitate the implementation of targeted management policies, more effective pollution control measures, and, ultimately, the reduction of diseases attributable to particulate pollution. (R.A.)