Prevention of exacerbations of chronic obstructive pulmonary disease (COPD) can involve removing the cause or reducing the patient's vulnerability to the cause. This article addresses the following issues: What is the problem during an exacerbation, what are the causes of an exacerbation, what can prevent exacerbations, and who are we? The difference between a patient with COPD during an exacerbation and after recovery is small. It is unlikely that patients with early COPD experience less exposure to exacerbation causes than those with severe disease; it is just that the consequences are more severe for those with severe disease. Interventions that produce small absolute benefits can therefore have a disproportionately large effect on exacerbation reduction. Recognized causes include season, cold weather, pollution events, bacterial infection, viral infection, and treatment withdrawal. Countries with warmer climates have much larger mortality in cold weather than those with colder climates. Reducing exacerbations in more temperate climates may be altered as much by changes in clothing and bedroom heating as by changes in treatment. Taking more exercise in cold weather may be the underlying reason for the reduction of exacerbations after pulmonary rehabilitation. Influenza vaccination reduces influenza severity and reduces transmission from health care workers to patients. There are a number of pharmacologic interventions shown to reduce (the effect of) exacerbations, including inhaled corticosteroids, long-acting beta-agonists, long-acting anticholinergics, mucolytics, and perhaps antibiotics that reduce Haemophilus carriage. The effect of the bronchodilators is additive to inhaled corticosteroids; how far the other interventions are complementary is unclear. So far, we have had a very medical response to COPD exacerbations. Altering social and behavioral aspects is likely to be complementary.