The main aim of this thesis is to describe the changes which are detectable on intravascular ultrasound and magnetic resonance imaging in patients with pulmonary hypertension and to determine whether these imaging modalities could be of use for the clinical assessment of the condition. Intravascular ultrasound was performed in 10 young adults with Eisenmenger's Syndrome and 4 infants with pulmonary hypertension secondary to a left to right shunt. Vasodilator studies were performed in 5 of the patients with Eisenmenger's. The vessel wall appeared as a single echogenic layer in all patients making it difficult to define or measure medial thickness with certainty. Morphological changes of intimal hypertrophy and atherosclerosis were evident in patients with Eisenmenger's whereas in the infants the intima appeared thin and smooth, typical of normal artery. The technique gave excellent definition of the vessel lumen allowing continuous measurement of changes in luminal dimensions in response to vasodilators. MRI of the pulmonary arteries was performed in 11 patients with Eisenmenger's and 6 normal controls. In patients with pulmonary hypertension the pulmonary arteries were found to be dilated with reduced distensibility when compared with normals. Calculations of Qp:Qs by MRI in patients with systemic to pulmonary shunts and pulmonary hypertension did not correlate well with values from cardiac catheterisation in all patients. In conclusion, magnetic resonance imaging was found to have limited role in the assessment of pulmonary hypertension but with new technical developments could become a non-invasive method of studying pulmonary hypertension in the future. The morphological changes detectable by intravascular ultrasound tend to be in severe disease only but the technique provides a unique method of studying pulmonary vascular reactivity in life.