The Marathon dam was built by American contractor Ulen & Company from 1926-1929 to provide water for Athens. The water was delivered to the Galatsi water treatment plant through the 13*4 km long Boyati tunnel and was an important milestone in the technological advancement of modern Greece. This gravity dam is 54 m tall and 285 m long and impounds a reservoir with a capacity of 41 000 000 m3, and was one of the largest construction projects in Europe at the time. Designed and built by a firm based in New York, the Marathon dam was by no means a purely American product: Greek engineers played key roles in all aspects of its planning, design and construction. The history of the Marathon dam thus illustrates that large engineering projects in countries deemed to be located on the edge of the developed world are seldom mono-faceted processes comprising an active 'transmitter' and a passive 'receptor'. The authors conclude that new insights and substantial reappraisals of engineering projects can emerge if historians of technology consider them as examples of active cooperation between technologically more advanced and less developed nations, rather than assuming they consist only of one-directional technological transfer.