This doctoral thesis aims to investigate the demographics, treatment patterns and prognosis of paediatric pulmonary hypertension (PH) and the emerging group of children and adults with PH in the setting of repaired congenital heart disease (CHD). I have conducted three studies, each with a distinct focus. The first is a retrospective, longitudinal study of ten CHD centres across the UK that assesses the clinical characteristics and treatment patterns of adults with a Fontan-type circulation receiving pulmonary vasodilators. I have compared treated patients with a matched cohort of Fontan patients who are not receiving pulmonary vasodilator therapy, complemented by an expert survey to determine current practice and the goals of therapy. In the second study, I created a 20-year national UK registry of paediatric PH and derived estimates of incidence and prevalence for all groups of paediatric PH in different age categories. I determined the natural history of paediatric PH and performed survival analysis. I then focused on patients with CHD and described the changes in demographics, with a substantial increase in patients with previously repaired CHD, who now form the largest PAH-CHD subgroup. The third study focuses on this latter group of repaired PAH-CHD. I highlighted the heterogeneity in terms of severity and onset of PAH. I identified prognostic markers and variables associated with PH resolution and developed a novel risk score for predicting adverse clinical outcomes in this group. This score will form the basis for the risk stratification of this high-risk population, to inform prognostication and guide treatment.