This thesis explores the career of Lord Broghill, one of the most important politicians of the Civil War and Interregnum period. The first chapter looks at his political 'apprenticeship' in the late 1630s and early 1640s, and his place in the Boyle dynasty, which had a profound affect on his later career. Chapters two and three consider his experience of the Irish rebellion in the 1 640s; his involvement in the factionalism of his home province of Munster; and his contact with the political parties at Westminster. Chapter four evaluates Broghill's relationship with Oliver Cromwell, and charts his rise to the head of a distinct Irish Protestant party, with its own coherent political strategy for the 'settlement' of all three nations. From 1655 until 1656 Broghill was president of the council of Scotland, which is the subject of chapter five. There he tried to create a moderate, civilian administration, and to make compromises with the 'Resolutioners' within the Scottish Kirk. His activities can be seen to parallel the attempts of Henry Cromwell to bring stability to Ireland. In chapter six there is an attempt to step back from the narrative, and to examine two important influences on Broghill's political career in the 1650s: the perilous state of his finances, and the strength of his religious convictions, both of which encouraged him to push for 'settlement'. Chapter seven provides a detailed analysis of Broghill 's role in the 1656-7 parliament, his importance in framing the Humble Petition and Advice and the offer of the crown to Cromwell, and his skill in marshalling English, Irish and Scottish MPs in support of his reform programme. With so much at stake, Cromwell's refusal of the crown was a personal as well as a political disaster for Broghill and his allies. Chapter eight, which takes the form of an epilogue, follows Broghill's decline in influence and enthusiasm at the end of the decade, and suggests reasons for his support of the Restoration in 1660.