How do we know that a development NGO is effective? Consultants writing evaluation reports typically describe what an NGO has done and how outcomes have changed over time, implying that such changes can be attributed tothe NGO. This was never convincing but only recently has the disenchantment with the before-after comparisons in most evaluation reports become widespread. There now is a surge of interest in using rigorous impact evaluation methods such as randomized control trials (RCTs) to assess what works in development.