Magnetic resonance imaging (MRI) offers the unique ability to examine the heart in three dimensions and to provide tissue characterisation. This thesis aims to investigate two areas of particular interest, namely the quantification of cor pulmonale and acute myocardial infarction. The methods currently available for assessing right ventricular hypertrophy and dilatation are discussed, with particular reference to patients with chronic obstructive pulmonary disease. The results of pulmonary haemodynamic and blood gas data are compared with the results from cardiac gated MRI. A good correlation is obtained between the mean and systolic pulmonary artery pressures, the pulmonary vascular resistance and the right ventricular wall volume measured by MRI. This technique is subsequently used in a small group of patients, to determine the response to long term oxygen therapy. The various techniques employed to assess the size of myocardial infarcts are discussed, and the previous literature on the ability of MRI to detect infarction is reviewed. A new technique, which is supported by phantom experiments, is described to measure the volume of infarcted myocardium using saturation recovery - inversion recovery and spin-echo images. The T1 images are compared with radionuclide pyrophosphate scanning and serum creatine kinase-MB release and a satisfactory agreement obtained. Myocardial infarct size measured by both pulse sequences is compared directly. The spin-echo technique is used to assess alterations in myocardial infarct size with time and the response to various therapeutic options is compared. In conclusion the limitations of both techniques are discussed and future developments proposed.