Cardiopulmonary exercise testing allows the assessment of the integrative cardiopulmonary response to exercise and is a useful tool to assess the underlying pathophysiologic mechanisms leading to exercise intolerance. Patients with pulmonary hypertension often face a considerable delay in diagnosis due to the rarity of the disease and nonspecific symptoms of dyspnea, fatigue, and exercise limitation. Cardiopulmonary exercise testing may be suggestive of pulmonary hypertension in patients with evidence of both circulatory impairment and ventilatory inefficiency. Other factors, such as mechanical ventilatory constraints from dynamic hyperinflation and peripheral muscle dysfunction, contribute to the profound dyspnea during exercise experienced by many patients with pulmonary hypertension. In patients with pulmonary arterial hypertension or chronic thromboembolic pulmonary hypertension, several exercise variables, such as low peak Symbolo2, high Vd/Vt, and high Symbole/Symbolco2, have proven to be useful in establishing the severity of functional impairment, predicting prognosis, and assessing the efficacy of interventions.