Cracking of concrete at Owen Falls power station, Uganda, was diagnosed as being due to concrete expansion caused by alkali-aggregate reaction. Resulting structural movements had caused local overstressing and also deflections of installed plant and equipment. The degree of expansion varied according to the different cements used during construction. The processes of detecting and clarifying the implications of the various movements are explained, together with measures taken to mitigate immediate problems and provide adequate monitoring to areas of longer-term concern. Lessons are drawn for the guidance of others investigating similar phenomena.