Core executive decision-making in economic policy in the UK is dominated by a Prime Minister-Chancellor axis and a set of constraints defined by vast flows of capital around foreign exchange markets. This thesis examines policy-making during the Thatcher governments in relation to the debate about ERM membership from 1979 to 1990. The analysis reconstructs the choices which faced the Thatcher governments given their economic and European policy interests and capital accumulation priorities, and investigates core executive actors' activity against this background. From the first Thatcher administration onwards, the core executive was seriously divided on ERM membership and the government was unable to pursue a coherent policy on the issue. As a result of both a power struggle between the Prime Minister and successive Chancellors and the retention of empirically untenable policy positions by core executive actors, economic policy-making failed as a judgement about effective means to ends. In this sense, decision-making became non-rational. Having renounced the potential benefits of ERM membership for most of the 1980s, the Prime Minister and Chancellor decided to enter ERM in autumn 1990 at a central rate of DM2.95 which served neither their own interests nor those of UK producers. The failure of the Conservative government to pursue an effective policy on ERM membership represented a failure to cope with or understand the implications for successful economic management of vast capital flows around foreign exchange markets.