Pulmonary hypertension (PH) is a common complication of advanced chronic obstructive pulmonary disease (COPD) and is defined by a mean pulmonary artery pressure (PAP) ≥ 25 mm Hg at rest in the supine position. Owing to its frequency, COPD is a common cause of PH; in fact, it is the second most frequent cause of PH, just after left heart diseases. PH is due to the elevation of pulmonary vascular resistance, which is caused by functional and morphological factors, chronic alveolar hypoxia being the most important. In COPD PH is generally mild to moderate, PAP usually ranging between 25 and 35 mm Hg in a stable state of the disease. A small proportion of COPD patients may present a severe or "disproportionate" PH with a resting PAP > 35-40 mm Hg. The prognosis is particularly poor in these patients. In COPD PH worsens during exercise, sleep and severe exacerbations of the disease, and these acute increases in afterload may favour the development of right heart failure. The diagnosis of PH relies on Doppler echocardiography, and right heart catheterization is needed in a minority of patients. Treatment of PH in COPD relies on long-term oxygen therapy (≥ 16h/day) which generally stabilizes or at least attenuates the progression of PH. Vasodilator drugs, which are commonly used in idiopathic pulmonary arterial hypertension, have rarely been used in COPD, and we lack studies in this field. Patients with severe PH should be referred to a specialist PH centre where the possibility of inclusion in a controlled clinical trial should be considered.