Acute exacerbation of chronic obstructive pulmonary disease (COPD) is defined by modification of the usual COPD symptoms, dyspnea, coughing and sputum, beyond daily variations, with a sudden onset, and requiring modification of the usual treatment. Exacerbations stud the course of COPD. Their frequency is variable, averaging 1-2 per year. Their frequency generally increases with COPD severity. Exacerbations impair patients' quality of life and aggravate disease prognosis by accelerating the decline in FEV1, the primary indicator of respiratory function. The most frequent causes of exacerbations are viral and bacterial respiratory infections and pollution. No cause is identified for nearly one third of all exacerbations. Most exacerbations can be treated at home, if a careful search for signs of clinical severity is negative. Treatment combines inhaled bronchodilator agents (beta-2 agonists, combined if necessary with anticholinergics) and oral corticosteroid therapy (prednisone: 0.5 mg/kg/d for 1 week) when the COPD is severe or signs of severity accompany the exacerbation. Antibiotic therapy is justified when the sputum appears purulent. Severe exacerbation may require oxygen therapy in cases of severe hypoxemia (PaO(2)<60 mm Hg) or mechanically assisted ventilation, essentially by noninvasive ventilation in cases of respiratory acidosis (pH<7.35). Noninvasive ventilation improves dyspnea and respiratory acidosis, diminishes respiratory frequency, intubation, duration of hospitalization, nosocomial infections, and mortality. Pulmonary follow-up is necessary after an exacerbation, especially to prevent the recurrence of exacerbations by measures that have been demonstrated to be effective, including help in smoking cessation, adaptation of COPD treatment, vaccination against influenza and pneumonia (pneumococci), and respiratory rehabilitation. Early diagnosis and rapid treatment of exacerbations can limit their impact, improve quality of life, and reduce the risk of hospitalization.