Prevalence of COPD is 4 to 10% in developed countries according to spirometric studies. COPD is markedly under-diagnosed, only 1/3 of patients being recognised. COPD is the only disease whose rank among causes of death increases, placing it at the third place in 2020. In France, COPD-related annual mortality rate is 25 per 100 000, which corresponds to 15 000 deaths. Between 1990 and 2020, it will rise from the 12th to the 5th rank among causes of handicap in the world. The main risk factor is tobacco smoking. Other factors with proven responsibility in the occurrence of COPD include some occupational exposures and inherited alpha 1-antitrypsine deficiency. Other genetic factors, prematurity, bronchial hyper-responsiveness and respiratory infections may also be involved.