Aid effectiveness has long been disputed, after decades of inconclusive macroeconomic analysis. Now there is a growing body of evidence from detailed, field level, microeconomic impact evaluations. The articles in this IDS Bulletin show how the design of these studies increasingly address the various sources of bias for which previous projects were criticised. These later evaluations provide a firm basis on which to draw conclusions on aid effectiveness. Generalisations – bearing in mind specific contexts in which interventions have or have not worked – will come from further evaluations. This volume presents examples of these studies from a number of agencies: AFD, ADB, IDB, JBIC, the Netherlands Ministry for Foreign Affairs, USAID and the World Bank.