There is no generally accepted definition of an exacerbation either for asthma or for chronic obstructive pulmonary disease. There is little consistency among the symptomatic or functional criteria used in different studies. The most consistent criterion is the introduction of systemic corticosteroids for the acute worsening of the disease. The time course of an exacerbation does not seem to differ very much between asthma and chronic obstructive pulmonary disease (COPD). The decrease in peak flow rate is more pronounced in asthma than in COPD. The frequency of exacerbations is linked to disease severity both in asthma and COPD. Common causes are viral infections and increased environmental air pollution, whereas allergen exposure and bacterial infections are more specific for asthma and COPD exacerbations, respectively. Few data are available about the airway pathology of asthma or COPD exacerbations. Eosinophilia and/or neutrophilia have been associated with exacerbations in both diseases. Avoidance of the causal factors decreases exacerbation rate in both diseases. Pharmacologic prevention of exacerbations in asthma has been shown for inhaled corticosteroids, combination therapy with long-acting inhaled beta(2)-agonists and inhaled corticosteroids, and monoclonal anti-IgE. Inhaled corticosteroids, long-acting inhaled beta(2)-agonists, combination therapy with both, and the long-acting inhaled anticholinergic tiotropium decrease the exacerbation rate in COPD.