Pulmonary arterial hypertension (PAH) is commonly associated with adult congenital heart disease. Eisenmenger syndrome-severe pulmonary hypertension with shunt reversal and cyanosis-represents the extreme manifestation of PAH in patients with congenital heart disease and has become the epitome of PAH in this setting. Despite representing a multi-system disorder and being associated with numerous complications, survival prospects for patients with Eisenmenger syndrome are superior compared with patients with idiopathic pulmonary hypertension. We review aetiology, clinical presentation, and prognosis of Eisenmenger syndrome and discuss potential explanations for the remarkable resilience of the right ventricle in this setting.