The link between exposure to air pollution and exacerbation of asthma symptoms has been investigated by epidemiological study and by direct biological experimentation. In asthmatics, epidemiological studies generally show a positive correlation between the particulate fraction of air pollution and increased morbidity, although roles for other co-pollutants (for example, ozone) are implicated as well. Direct experimentation using air pollutants, especially particles, to investigate their effects on humans or on animal models of asthma provides corroboration of the epidemiology and has begun to identify the pathophysiological mechanisms involved. We begin this review with an overview of air pollution, followed by a survey of the epidemiological and experimental data regarding air pollution particles and asthma. We finish with a discussion of directions for future research.