Asthma exacerbations are common, and the major morbidity, mortality, and health care costs associated with asthma are related to exacerbations. The majority are related to viral infection, and although progress has been made in identifying the mechanisms of virus-induced asthma exacerbations, there is still much to be learned. Allergen exposure causes some exacerbations and can participate in virus-induced exacerbations, as can other environmental exposures. A role for atypical bacterial infection in exacerbations is also increasingly recognized. Exacerbations are characterized by airway inflammation, which can differ in type depending on whether it is primarily infective or allergic in origin. An increased understanding of the inflammatory pathways might lead to identification of targets for the development of novel prevention or treatment strategies.