The authors describe how in the design of the Clywedog and Craig Goch dams, structural geology was of great importance. At Clywedog there was a major mineralized fault, deeply mined, a short distance downstream of the dam site and this represented a structural discontinuity in the dam and foundation complex. At Craig Goch suspected faults or damaged zones had to be identified and dealt with. The definition of the superimposed joint and fissure pattern in the gorge was essential.