Pulmonary hypertension (PH) is defined by a mean pulmonary artery pressure (PAPm) superior than 25mmHg at rest or superior than 30mmHg with exercise. The classification of PH differentiates between "secondary" PH which results from a well-known disease, such as PH due to thromboembolic disease (obstructive PH), left cardiac failure (passive PH), or chronic respiratory diseases (hypoxic PH), and pulmonary arterial hypertension (PAH). PAH is a rare disease characterized by a progressive increase of pulmonary vascular resistance leading to right ventricular failure. PAH is classified as idiopathic, familial, or associated with various conditions (connective tissue diseases, congenital heart diseases with systemic-to-pulmonary shunts, portal hypertension, infection with the human immunodeficiency virus, or appetite-suppressant drugs). Transthoracic Doppler echocardiography is the investigation of choice for non invasive detection of PAH but right-heart catheterization is necessary to confirm the diagnosis of PAH and determine its mechanism. Pulmonary function tests and chest CT scan may detect an underlying chronic pulmonary disease (hypoxic PH). Lung perfusion scan and contrast-enhanced chest spiral CT scan can lead to the diagnosis of thromboembolic PH, which is to be confirmed by pulmonary angiography. Assessment of the severity of PH is based on clinical parameters (NYHA, right heart failure), functional tests (six-minute walk test), echocardiography and hemodynamics. Characterization of PH is essential in the management of PH because it determines the appropriate treatment: an etiological treatment in passive, obstructive or hypoxemic PH, or vasodilatator and antiproliferative therapies in PAH.