An impact evaluation identifies the long-term changes that can be attributed to a project, programme or policy. In its most common form, it simulates what would have happened to the beneficiaries had they not participated in the project, programme or policy --the so-called "counterfactual". There are also impact evaluations that indicate what contributions different actors made to the observed impact. In all cases, proper impact evaluation explains how long-term transformations were produced, describing the change mechanisms in addition to measuring differences.