This publication aims to support impact evaluation in the transport sector by assessing what has been produced against what might be possible. It reviews 91 impact evaluations of transport interventions in developing countries, summarizes findings on outcomes, identifies evidence gaps, and proposes ways forward. Most of the studies reviewed find significant effects on at least one of the outcomes investigated. However, impact evaluation has given relatively little coverage to major areas of investment, such as urban and sustainable transportation, transport corridors, and efficiency enhancing measures. New methods and increasing openness of geospatial data provide scope to generate more innovative impact studies.