Prevailing economic ideas -- and fashions -- about development have influenced the International Development Association (IDA) since its creation in 1960. The creation of the organization itself is the result of two contemporaneous facts: an urgent need to channel development finance to least-developed countries and an increasing pressure on World Bank management to directly address the issue of poverty in developing countries. Changing views, over time, have been a rationale -- and, at times, a justification -- for emphasizing poverty and social sectors; for providing grants to particular groups of countries; and for strategic choices and sectoral priorities. IDA has been influential in development debates and been an advocate for specific views about development policy. This paper gives an overview of these views and documents how they have shaped the activities of the organization since its creation. After a brief review of development thinking and of the organization of research at the World Bank, the paper documents the shifts that have taken place in country allocations and in sector emphasis in IDA over the past 50 years and highlights the strategic themes that have guided its development agenda: toward increasing country selectivity; from projects to programs; from conditionality to country ownership of reforms; and from input-based to results-based performance.