Trust funds – broadly defined as financial vehicles to channel development funding earmarked for specific purposes through international development organizations – have grown substantially over the past two decades. Reflecting the variety of trust fund purposes and related governance arrangements, an emergent literature emphasizes a diversity of reasons underlying this growth. This paper proposes a simple – yet encompassing – explanation applicable to all kinds of funds: donors use trust funds to wield ‘influence’ – leveraging financial resources to alter the policies of multilateral organizations. Based on interviews at the World Bank, the study shows that influence is a dominant motive behind trust funds, though the capacity and willingness to wield influence varies across donors. Influence is a salient motive especially for medium-sized donors and emerging donors but surprisingly less so for large donors. In addition, attempts of influence are most effective when donors promote new thematic issues that did not previously feature Bank assistance and outside established programs. Concerns among stakeholders about undue donor influence are highest with respect to the global knowledge work of the World Bank but are virtually absent when involving donors in the operational activities at the country level.