Results-based aid (RBA) is a form of foreign assistance in which one government disburses funds to another for achieving an outcome. This paper distinguishes four different theories used to justify RBA programs and analyzes four case studies – from GAVI, the Amazon Fund, Ethiopian Secondary Education and Salud Mesoamérica. The analysis shows that relatively few RBA programs are being piloted and that few adverse effects have materialized. Rather, these initiatives are relatively cautious and unable to test the potential benefits from greater recipient discretion and public transparency. Future experimentation may remedy this but for now, RBA remains a work in progress.