Calls for rigorous impact evaluation have been accompanied by the quest not just to find out what works but why. It is widely accepted that a theory-based approach to impact evaluation, one that maps out the causal chain from inputs to outcomes and impact and tests the underlying assumptions, will shed light on the why question. But application of a theory-based approach remains weak.

This paper identifies the following six principles to successful application of the approach:

1. Mapping out the causal chain (programme theory)
2. Understanding the context
3. Anticipating heterogeneity
4. Rigorous evaluation of impact using a credible counterfactual
5. Rigorous factual analysis
6. Using mixed methods