Chronic obstructive pulmonary disease (COPD) is characterized by chronic airflow obstruction, leading to an airflow limitation that is not fully reversible, with extrapulmonary effects or comorbidities that may contribute to the severity of the disease. The most important cause is tobacco smoking that is associated with an abnormal inflammatory pulmonary and systemic response. Chronic airflow obstruction is defined by postbronchodilator FEV1/FVC ratio < 0.7. A spirometric classification of the COPD severity is recommended into four stages. COPD has a variable natural history. Usually, COPD is a progressive disease with an increase in airways obstruction. Airflow limitation leads to hyperinflation, the most important cause of dyspnea in COPD patients. Assessment of the prognosis is based on the severity of the spirometric abnormality (FEV1) and includes other factors such as body mass index (BMI), dyspnea and exercise impairment. The underlying disease process in COPD leads to gas exchange abnormalities, chronic respiratory insufficiency and pulmonary hypertension. The more frequent causes of death are respiratory insufficiency, cardiovascular comorbidities and lung cancer.