On average, 400,000 US dollars and a period of three years is needed to measure the effect of a development intervention, the International Initiative for Impact Evaluation concludes from its activities. These are valuable resources that need to be used carefully. Rural 21 (the International journal for rural development) describes which impact measurement methods have proved to be useful during the last few years and what their strengths and their limits are. And its authors demonstrate when impact evaluations make sense – and when they don’t.