Richard, Duke of Gloucester (later Richard III) was appointed to a number of the highest offices of the realm by his brother King Edward IV, of which one of primary importance was Lord High Constable of England. He retained this office in 1483 while concurrently designated Protector and Defender of the Realm during the minority of Edward V. For a crucial few weeks Gloucester combined in his person two offices whose significance has been consistently misunderstood and overlooked. In effect he held overarching responsibility for defence of England against enemies from outside and rebels within, while exercising summary jurisdiction over the crime of treason. Presented in two parts, the book begins with the origins and development of the separate offices of Protector and Constable, considering their principal 15th-century incumbents and their place in the constitutional framework. It summarizes the laws governing the crime of treason, and indicates how successive monarchs, notably Edward IV, placed increasing reliance on the High Constable and his Court to deliver judgement and sentence in such cases. In the second part, this study addresses the misapprehensions (at the time and in subsequent centuries) about Gloucester’s responsibilities and powers, and examines the principal events during his protectorate in the context of both offices, viewed from the perspective of 15th-century precedent rather than 16th-century hindsight.