The diagnosis of chronic obstructive pulmonary disease (COPD) relies on the presence of chronic airflow limitation poorly reversible or not reversible at all, defined by an FEV1/FVC ratio less than 70%. Stages of severity of COPD are defined according to the level of post-bronchodilator FEV1: > 80% of the predicted value (stage I); 50-80% (stage II); 30-50% (stage III); < 30% (stage IV). Accordingly, the measurement of pulmonary volumes (spirometry) is required for the diagnosis but also for the follow-up of COPD patients. The investigations which are required depend on the severity of COPD: spirometry and flow-volume curves during forced expiration are sufficient in stage I; measurement of static lung volumes and bronchodilator reversibility testing are required in stage II. Arterial blood gases should be measured in stages III and IV. Pulse oxymetry and 6minute walk test (6MWT) are recommended from stage II. For appreciating the severity of COPD and for the follow-up of patients it is recommended to evaluate other variables than FEV1: results of the 6MWT, level of dyspnea, body mass index. The results of FEV1 and of these variables are included in the recently developed BODE index. Measurement of CO transfer capacity is recommended in the presence of emphysema; cardiopulmonary exercise testing (bicycle) is recommended before initiating exercise training.