Over the past 50 years, the World Bank has increasingly relied on resources contributed voluntarily from donors and held separately from its core budget to support projects and activities, particularly for global health.1 These resources are known as trust funds. In the case of the bank, these trust funds are synonymous with earmarked, extra-budgetary, and “multi-bi” aid (bilateral aid channelled through multilateral institutions).2 3 The absolute number and relative proportion of bank assets held in trust has skyrocketed since the early 1990s. In 2011, the bank was trustee to roughly half of the trust funds for official development assistance (ODA) worldwide,4 5 and in 2012-13 about 200 donors contributed $3.7bn to more than 1000 World Bank Group trust funds.6