Pulmonary hypertension ( PH ) was previously classified into 2 categories: primary pulmonary hypertension and secondary pulmonary hypertension according to the presence of identified causes or risk factors. After the second World Symposium on pulmonary hypertension (WSPH) held in 1998, a clinical classification of PH was established in order to individualize different categories of PH sharing similar pathological findings, similar hemodynamic characteristics and similar management. Five groups of disorders that cause PH were identified: (Group 1): Pulmonary arterial hypertension; ( Group 2 ): pulmonary hypertension due to left heart disease; ( Group 3):pulmonary hypertension due to chronic lung disease and/or hypoxia;( Group 4): chronic thromboembolic pulmonary hypertension ; ( Group 5 ): pulmonary hypertension due to unclear multifactorial mechanisms. During the successive world meetings, a series of changes were carried out, reflecting some progressions in the understanding of the disease. However, the general architecture and the philosophy of the clinical classification? were unchanged. During the 5 th WSPH held in 2013. The consensus was reached to maintain the general scheme of previous classification. However modification and update especially for Group 1 patients were proposed. The main change was to withdraw persistent pulmonary hypertension of the newborn (PPHN) from Group 1 because this entity? carries? more difference than similarities? with other PAH subgroups. In the current classification, PPHN is now designated number 1. Pulmonary hypertension associated with chronic hemolytic anemia has been moved from Group 1 PAH to Group 5.unclear mechanism. The congenital or acquired left heart inflow/outflow obstructive lesions and congenital cardiomyopathies have been added to Group 2 and segmental pulmonary hypertension has been added to Group 5. Such changes related to pediatric pulmonary hypertension was decide in order to create a comprehensive, common classification for both adult and children.There were no changes for Group 2.3 and 4. The Group I PAH include idiopathic PAH, heritable PAH, Drug or toxin induced and PAH associated with specific disorder: connective tissue disease, HIV infection, portal hypertension and congenital heart disease in adult, schistosomiasis. The Group 2 PH include left ventricular systolic dysfunction, left ventricular diastolic function, valvular heart disease and Left heart inflow/outflow obstruction and congenital cardiomyopathy. Pulmonary hypertension is defined by a mean pulmonary artery pressure > 25 mmhg at rest, measured during right heart catheterization. PAH describes a subpopulation of patient with PH characterized hemodynamically by the presence of pre-capillary PH including an end-expiratory pulmonary artery wedge pressure < 15 mmhg and pulmonary vascular resistance > 3 Wood unit.Right heart catheterization remains essential for diagnosis of PH or PAH.