This thesis describes and examines the Earl of Sussex's performance as a royal servant. After gaining some military, diplomatic and administrative experience, in 1556 Sussex was appointed Lord Deputy of Ireland. There he attempted to implement the royal policy of plantation in Leix and Offaly and 'surrender and regrant' in the Irishry. However, unable to tame or defeat the Ulster chieftain, Shane O'Neill, and accused of maladministration of the Pale, Sussex was recalled and replaced by Sidney in 1565. The chapters on Ireland have studied the complex Irish wars and Sussex's ideas for the pacification of the area and investigated the nature and truth of the accusations made against him. • After his return to Court, Sussex collaborated with Cecil and Norfolk to promote a marriage between Elizabeth and Archduke Charles of Austria; to such good effect that in 1567 he was sent to Vienna to negotiate a settlement. The Habsburg marriage scheme was instrumental in the formation of factions at Court. The resultant conflict has been examined as well as the progress of the negotiations and Sussex's arguments in favour of the marriage.