The focus on results in development agencies has led to increased focus on impact evaluation to demonstrate the effectiveness of development programmes. A range of methods are available for counterfactual analysis of infrastructure interventions, as illustrated by the variety of papers in this volume. Understanding impact means understanding the context in which an intervention takes place and the channels through which the impact on outcomes is expected to occur. Such analysis typically requires mixing both quantitative and qualitative approaches. The analysis will also anticipate heterogeneity, with conditioning for ‘selection bias’ being recognised as positive information about for whom and when an intervention works or not.