This thesis is a chronological case study into the origins, operation and consequences of the Lib-Lab Pact 1977-1978. Cross-party co-operation in British politics since 1945 is assessed. David Steel's election as Liberal Party leader, his political philosophy and strategy are examined. Concepts of realignment, 'co-operation strategy' are explored. The parliamentary and political events together with a detailed assessment of the inter-party negotiations which led the Pact are examined. New perspectives include: the significance of the leader-led nature of the negotiation process; the Labour-Ulster Unionist understanding which ran concurrent with the Pact; the importance of Lib-Lab discussions on devolution which pre-dated the Pact in influencing Steel's subsequent decision-making. Analysis focuses on the Lib-Lab negotiations into if the Direct Elections to the European Parliament Bill should include a proportional voting system and whether the parliamentary Labour party should be compelled to vote for PR. A key finding of the thesis is that rather than allowing a free vote, as was agreed, the Prime Minister, James Callaghan, was prepared to offer the Liberals a 'pay roll' vote; the significance of Michael Foot in this process is also noted. The structure of the Lib-Lab consultative mechanism is reviewed. Case studies include a review of on Liberal policy influence on the Budget 1977 and 1978. The nature of intra-party dissent is reviewed with the difference between Labour and the Liberal parties noted. An examination of the serious internecine conflict is complemented by a reassessment of the role of Christopher Mayhew in this process. The Lib-Lab Pact is reviewed, assessing its affect in influencing Callaghan's decision not to call a General Election in 1978; its influence on Liberal/Liberal Democrat party strategy, and its importance in the subsequent formation of the triple-lock, as such the thesis highlighting the Pacts relevance to subsequent cross-party understandings.