This thesis is primarily based on the study of English diplomats from 1375 to 1422, and provides a detailed examination of the individuals, their social and geographical backgrounds, and their experience and careers in order to constuct an overall picture of the men who represented English kings abroad. A prosopographical approach is used to reconstruct the underlying social networks and patterns of patronage, to examine whether diplomats were chosen purely for political reasons or whether an element of increasing professionalisation can be discerned. To this end, the final section of the thesis explores diplomatic practice when on embassy, and considers how and why the choice of diplomat and the work he undertakes varies according to the country to which he is sent. The research upon which this thesis is based is founded on the belief that those who enact royal diplomacy have been historiographically marginalised in favour of those who decide that policy, despite the influence that diplomats had on information exchange and negotiations during their service as mediators between the king and his fellow rulers. It moves beyond the traditional Anglo-French paradigm of medieval diplomatic historiography to examine Anglo-Continental relations in the period 1375-1422, thus including English relations with peripheral kingdoms such as Hungary, Norway and Sweden. Part One provides an explanation of some of the theoretical issues behind the use of prosopography and databases in history, in particular, how an approach based on providing statistically meaningful results is compatible with issues central to the work of medieval historians, such as documentary survival and incomplete data. The second section, the Iynchpin of the thesis, begins with a prosopographical analysis of background, families and social networks of diplomats from 1375-1422. It explores the geographical origins, patronage and the influence of early service in the households of, amongst others, the dukes of Lancaster and earls of Arundel - both of which served as 'incubators' for a number of future diplomats.