The article describes the piloting of impact evaluations in the context of GTZ independent evaluations. Object of the pilot-impact evaluations were three water sector programs in Zambia, Kenya and Turkey. First of all the article defines and describes the concepts of impact and how to measure impacts. Based on these clarifications, hypothesis for the measurement of the concerned program impacts are derived. Furthermore the evaluation method and its implementation, which covers hypothesis-guided cause-effect analysis, quasi-experimental design, multi-method approach and rigorous data analysis, are described. Finally, selected results and challenges in impact measurement are presented. The résumé of the article covers a critical assessment of the evaluation concept and its potential, to adequately measure impacts.