This article deals with the legal aspects of the dispute between Hungary and Slovakia concerning a joint project of locks and power-stations, which had to be constructed in the Danube at Gabc ikovo and Nagymaros. Rules of treaty-law, environ-mental protection, good faith, reprisal and contributory negligence will be applied to assess whether Hungary was entitled to terminate the treaty and whether Slovakia could lawfully construct a provisional solution. Furthermore the significance of the case and its submission to the International Court of Justice is addressed.