Research programmes in economics usually emerge from the intersection between a new analytical approach and a real economic problem. In the last few years, such a programme has emerged in international monetary economics, which is underpinned by a theoretical framework grounded in stochastic calculus and the increasing prominence in the real world of the international monetary arrangements under which national monetary authorities attempt to keep exchange rates within bands or 'target zones'. This new programme of research also covers switches in exchange rate regimes. This volume from the Centre for economic Policy Research and the National Bureau of Economic Research includes contributions - as authors or discussants - from most of the active participants in the development of this new field, and will serve as a useful introduction and basic text for this new research programme. It opens with an account of the basic economic model of a currency band developed by Paul Krugman, which is followed by two papers that extend this approach. Other chapters study the regime switches entailed in Britain's return to the gold standard in 1925 and the preannounced entry of a floating currency into a band such as the EMS. Essays on sustainability and realignment consider the possible outcomes of speculative attacks on such bands, and the volume ends with a paper on econometric testing of models of this type.