This Note defines impact evaluations, explains when they should be commissioned according to USAID policy and describes different designs for quasi-experimental and experimental impact evaluations. The USAID Automated Directives System (ADS) 203 defines impact evaluations as those that measure the change in a development outcome that is attributable to a defined intervention. Impact evaluations are based on models of cause and effect and require a credible and rigorously defined counterfactual to control for factors other than the intervention that might account for the observed change.