As the number of people affected by disasters has risen, so have the expectations placed on humanitarian agencies by donors, the public and the affected populations themselves. Agencies must now provide evidence of impact of their interventions. But applying conventional evaluation methods can pose problems. How can we assess the difference that intervention makes? Is it ethical to consign some disaster-affected communities to control groups? How feasible is it to collect baseline data among people who have just been traumatized? This guide provides a reliable and practical method for identifying the contribution an agency makes to changes to people's lives in the recovery period following disasters. It outlines 11 steps that take evaluators through designing quantitative and qualitative methods through to collecting field data and developing a narrative of evidence and change. The Contribution to Change tool encourages recognition of the efforts of the community itself, as well as the impact of other actors such as government or community-based groups. Contribution to Change should be read by humanitarian managers and field staff, as well as by researchers into humanitarian relief and international development.