In this article, we seek to explain reform patterns at the World Bank. Traditional realist and institutionalist theories say little about the process of change within international organizations ('IOs'). Drawing upon the insights of relatively new rationalist and constructivist approaches, we develop and test a model of IO change that combines insights from rationalism and constructivism. Our explanation integrates the 'top-down' logic of a rationalist principal-agent model – targeting the redesign of organizational structures, hiring procedures and promotional standards, and the 'bottom-up' logic of sociological constructivism – focusing on the transformation of bureaucratic culture. We find that reform outcomes hinge upon the ability of change entrepreneurs to disrupt both the logics of consequence and appropriateness that shape the preferences and behaviour of bureaucratic actors. We evaluate our model by examining four distinct aspects of the World Bank's Strategic Compact (1997-2001), which included attempts to alter project management, organizational culture, and the mission of the institution itself.