Exposure to ambient air pollution particles (PM) has been associated with increased cardiopulmonary morbidity and mortality, particularly in individuals with pre-existing disease. Exacerbation of pulmonary inflammation in susceptible people (e.g., asthmatics, COPD patients) appears to be a central mechanism by which PM exert their toxicity. Health effects are seen most consistently with PM with aerodynamic diameter < 2.5 micrometers (PM(2.5)), although 10 micrometers < PM < 2.5 micrometers can also be toxic. Through its metal, semi-quinone, lipopolysaccaride, hydrocarbon, and ultrafine constituents, PM may exert oxidative stress on cells in the lung by presenting or by stimulating the cells to produce reactive oxygen (ROS). In vivo, PM increase cytokine and chemokine release, lung injury, and neutrophil influx. In vitro analysis of PM effects on the critical cellular targets, alveolar macrophages, epithelial cells, and neutrophils, demonstrates PM- and oxidant-dependent responses consistent with in vivo data. These effects have been observed with PM samples collected over years as well as concentrated PM(2.5) (CAPs) collected in real time. Oxidative stress mediated by ROS is an important mechanism of PM-induced lung inflammation.