​Realist impact evaluation draws on the realism school of philosophy. Realist approaches to evaluation assume that nothing works everywhere or for everyone: context really does make a difference to programme outcomes. 

This introduction aims to explain realist impact evaluation for those unfamiliar with its principles. It describes five key ideas which frame realism and their implications for impact evaluation. It explains when a realist impact evaluation may be most appropriate and outlines how to design and conduct an impact evaluation based on a realist approach.

The paper will be useful for evaluators and commissioners of evaluations to decide whether a realist approach is appropriate or feasible for evaluating a particular programme or policy; as well as for anyone interested in understanding a realist approach to impact evaluation.