The Geneva WHO meeting was devoted to primary PH, a severe form of PH, some years after an outbreak related to the intake of the anorexic drug aminorex. Since the 1st World Symposium on Pulmonary Hypertension (WSPH) organised by the WHO in Geneva in 1973, PH has been defined as mPAP ≥25 mmHg measured by right heart catheterisation (RHC) in the supine position at rest. In 1961, a report of the World Health Organization (WHO) Expert Committee on Chronic Cor Pulmonale mentioned clearly that the mean pulmonary arterial pressure (mPAP) does not normally exceed 15 mmHg when the subject is at rest in a lying position, and that the value was little affected by age and never exceeded 20 mmHg. Regarding clinical classification, the main Task Force changes were the inclusion in group 1 of a subgroup “pulmonary arterial hypertension (PAH) long-term responders to calcium channel blockers”, due to the specific prognostic and management of these patients, and a subgroup “PAH with overt features of venous/capillaries (pulmonary veno-occlusive disease/pulmonary capillary haemangiomatosis) involvement”, due to evidence suggesting a continuum between arterial, capillary and vein involvement in PAH. Prospective trials are required to determine whether this PH population might benefit from specific management. Thus, this 6th WSPH Task Force proposes to include pulmonary vascular resistance ≥3 Wood Units in the definition of all forms of pre-capillary PH associated with mPAP >20 mmHg. However, this abnormal elevation of mPAP is not sufficient to define pulmonary vascular disease as it can be due to an increase in cardiac output or pulmonary arterial wedge pressure. This definition is no longer arbitrary, but based on a scientific approach. Two standard deviations above this mean value would suggest mPAP >20 mmHg as above the upper limit of normal (above the 97.5th percentile). Recent data from normal subjects has shown that normal mPAP was 14.0☓.3 mmHg. Since the 1st World Symposium on Pulmonary Hypertension (WSPH) in 1973, pulmonary hypertension (PH) has been arbitrarily defined as mean pulmonary arterial pressure (mPAP) ≥25 mmHg at rest, measured by right heart catheterisation.