Increasing evidence suggests that reactive oxygen species (ROS) and oxidative stress are involved in PM-mediated lung and cardiovascular injury. The physical characteristics and the chemical composition of particulate matter (PM) play a key role in ROS generation in vitro and in vivo. The mitochondria are major subcellular targets for PM as well as a source of ROS production. ROS production is due to interference in mitochondrial electron transfer and PT pore opening by pro-oxidative PM components. Another possible mechanism is direct physical targeting by ambient ultrafine particles that lodge in and destroy mitochondrial structure. An understanding of the mitochondrial effects of PM is key in understanding the mechanisms of PM-induced adverse health effects.