Hydroelectric power is one of the most widely exploited forms of renewable energy and, in areas where global climate change (GCC) causes precipitation characteristics to change significantly, or more importantly, where run off is reduced, existing hydroelectric installations may require replacement with alternative sources of power. As economic appraisals of new hydroelectric projects are normally made on a long-term basis, GCC may also necessitate revision of assessments of unexploited hydroelectric potential. This paper considers two geographic regions where significant climate change may occur and assesses how possible future precipitation distribution and water run-off may affect the viability of hydroelectric power.