The use of fly ash for controlling damaging alkali-silica reaction (ASR) was first reported in 1949 by Robert Blanks of the U.S. Bureau of Reclamation. 1 Since then, hundreds of papers have reported the results of laboratory studies on the efficacy of fly ash in this role. While many specifications now permit the use of potentially reactive aggregates, provided a sufficient level of fly ash (or other preventive measure) is used in the concrete, there have been relatively few documented cases of major structures where fly ash has been successfully used together with reactive aggregates. A paper on two such cases, the Nant-y-Moch Dam in Wales, U.K., and the Lower Notch Dam in Ontario, Canada, was published by the primary author when those facilities were about 35 and 25 years old, respectively. 2 The dams were revisited in 2010 when they were about 50 and 40 years old; this article summarizes the performance of these structures with regard to ASR. Studies of performance in the field are essential for confirming the efficacy of preventive measures observed in the laboratory and for benchmarking accelerated laboratory tests intended for the rapid evaluation of such measures. Nant-y-Moch Dam