This thesis, which is based on manuscripts in Magdalen College, Oxford, is a study of the English career of Sir John Fastolf (1380-1459). In the first chapter Fastolf's investment in land is examined and the conclusion is reached that Fastolf bought property in a limited and artificial market. The second chapter analyses the administration of Fastolf's estates. It demonstrates that Fastolf paid close attention to administration and policy-making. A survey of Fastolf's friends and associates, particularly in the light of the feoffments he made, is undertaken in the third chapter. The subject of the fourth chapter is the emnity between Fastolf and William, Duke of Suffolk during the 1440s. The high-handedness of the Duke and his officers is emphasised. The major lawsuits fought by Fastolf are studied in the final chapter. It is concluded that Fastolf suffered not because he had bought property unwisely but because he was the victim of the acquisitiveness of men who had political power during Suffolk's supremacy. Disputes, nevertheless, were almost always conducted in a peaceful way.