In early 1967, the Hashemite Kingdom of Jordan was among the first Arab states to reestablish diplomatic relations with the Federal Republic of Germany, leading to the development of an ever since continuing development cooperation programme. In consequence, GTZ/GIZ has been supporting the Jordanian water sector in tackling its many challenges for over 40 years: Between 1975 and 2016, a total of 30 projects were commissioned with an overall budget of more than 110 million Euros. The longevity and volume of this water portfolio give rise to questions regarding the development of the cooperation itself: What were the thematic key areas, and how did they evolve over time? What impacts were achieved, and could these be sustained until today? What are the lessons learnt?