Adoption of suitable impact assessment methodologies, which combine managerial insights with rigorous data analysis, are necessary for informed decision making and profitable results from the implementation of developmental projects. Four case studies are presented in this cover feature on Impact Assessment which shows that the results of such impact assessment exercises, themselves are inputs for future programmes. The social and economic development budget of India is well into hundreds of billions of dollars. Yet, when asked about the yardsticks used to measure success, policymakers often discuss developmental programmes – so many health camps held, so much concrete bought and so on. Rarely do they venture into the measurement and assessment of outputs, surely a critical exercise to understand the effectiveness of government programmes and policies. Such understanding, often termed impact assessment, is in itself an input into future policy decisions – should we continue with current programmes or discard them in favour of other programmes? Even if programmes are continued, can they be modified to maximise their effectiveness? And in case of completed programmes, what can we learn to carry over to new programmes?