Pulmonary hypertension (PHT) is a well recognised feature of untreated congenital heart disease. This article will review the causes, known mechanisms, appropriate investigations and current therapies for PHT. The reader will understand the difference between PHT due to high pulmonary blood flow and PHT that is due to high pulmonary vascular resistance. The former is best treated by surgical or catheter intervention, whereas for the latter (Eisenmenger syndrome) only palliation is possible with medication or transplantation. Echocardiography and electrocardiography (ECG) should be performed in any child where there is a possibility of pulmonary hypertension, especially with long standing chronic lung disease and minor left to right shunt. Often these children may have dual pathology and their investigation and management may be a complex interaction between cardiac and respiratory therapists. New treatments and new techniques of assessment are now available and this may lead to improved recognition of PHT and prevention of long term disability as a result.