Mullardoch Tunnel is a hydro-electric tunnel in the Highlands of Scotland, driven through metamorphic rocks of the Moine Series between 1947 and 1952. As with all large-scale hydro-electric projects, the geology exerted significant control and influence on the overall design and construction. However, although there was implicit and explicit consideration of geological aspects during the planning, design and construction of the schemes, the works largely predated the systematic application of engineering geology and rock engineering in the UK. Nonetheless, the geologists involved with the project were able to identify and effectively communicate the main geological issues relevant to the engineers. Given the continuing national import of hydro-electric schemes, this forms an interesting case study through which to retrospectively consider the geological input to hydro-electric tunnels. The geology, design, construction and maintenance of the tunnel are discussed together with the findings of recent inspections. Examples of good and bad practice are identified and implications for improved practice are drawn, including the continuing importance of high-quality geological input to engineering projects and the need for improved training in geological skills, particularly when applying modern practice to historical structures.