About 1 percent of the 9000 large dams in service throughout the world during the period 1900–1965 have failed and another 2 percent have suffered serious accidents. In more than 50 percent of these cases the damage could be related to geological causes. This Paper presents brief analyses of 9 projects which have been seriously affected by incidents related to geological causes and derives for each project a list of factors which are judged to have contributed to the incidents. It is shown that a significant number of incidents have occurred due to technological factors which were either unknown or inadequately understood at the time of design or construction of the project. It is concluded that there are undoubtedly still some unknown geotechnical factors in water storage engineering and that designs should continue to allow for this situation. The other contributing factors are mostly concerned with deficiencies in the application of geological science to civil engineering. To overcome such deficiencies an objective-oriented site investigation approach is outlined in which geological methods are applied together with engineering methods, to provide answers to various engineering questions. These questions mostly relate to possible changes to the site environment resulting from construction and operation of the project, and in particular to possible modes of failure of the project. It is considered important that geological data for water storage projects should be obtained in such a systematic manner and that the geological input be provided by experienced geologists with appreciable engineering knowledge. This knowledge may be gained partly from tertiary education institutions but should always include that gained from experience while working closely with geotechnical, design and construction engineers, on construction jobs.