This thesis is a study of the political party led by the second Marquess of Rockingham, from the perspective of the Upper Chamber of Parliament. The historiography of the Rockingham Whigs is extensive but underplays the House of Lords, while the literature on the House of Lords is growing, but lacks a major analysis of this significant opposition group. This is, therefore, a contribution to the debate on the nature of party in the eighteenth century. In Part I, the large number of surviving division lists and related parliamentary sources are analyzed, in order to provide a picture of the size and structure of the membership of the party in the House of Lords. In Part II, a session by session narrative of the debates in the Upper Chamber illustrates the leadership's attempts at parliamentary management, and shows how the Rockinghamite peers developed an increasingly coherent outlook on domestic, colonial and constitutional issues. Part Ill illuminates how Rockinghamite peers were caught between the conflicting poles of onerous political duties, and the attractions of local or particularist interests. On an analogy with their own bodily illnesses, and their conflated ideas of the diseased condition of the constitution, this dilemma is examined in terms of ease and 'dis-ease', in order to understand the party's collective mentality. What emerges is a complex portrait of the Rockingham Whig party in the House of Lords. At the centre there was a core of regular voters, a steady ideological out look, and a solid commitment to public life. But for the rest of the Rockingham Whigs there were great variations in the degree of their political allegiance, in the extent of their articulated affiliation to the leadership's policies, and in the continuity of their political motivations. Yet despite all the problems that they faced in opposition, the Rockingham Whig party succeeded in maintaining a well defined cohesion.