The value of geological information to an engineering project is at its greatest during the design phase and this situation is most clearly illustrated by the methods adopted in the location and design of reservoirs. The principles which determine watertightness are outlined and illustrated with regard to specific case histories. Attention is drawn to the differences between mass seepage through a reservoir flank and leakage through a defect in the rock structure over a more limited area. The methods which can be used to prevent or control groundwater flow are reviewed in relation to dam design and foundation geology. Apart from the question of watertightness, which arises in all cases of reservoir construction, the additional factors which determine reservoir feasibility, such as reservoir loading and seismic activity, and valley-side stability are examined. The geological information obtained early in project development and often at very limited cost may, therefore, be critical to fundamental engineering decisions which can influence the whole viability of the scheme.