This paper articulates how development assistance can promote program evaluation generally, and impact evaluation specifically, as a contribution to good governance. We argue that aid agencies are particularly well suited to fund impact evaluations, and can accelerate progress in the developing world by increasing the resources available for evaluation, particularly through a collective vehicle like the International Initiative for Impact Evaluation (3ie). Finally, we highlight the conditions that need to be in place – and require additional efforts – to yield the full benefits of collective investment in finding out what works.