Although humans have been damming rivers from historical times, construction of hydropower dams ebbed worldwide in the second half of the twentieth century. Global estimates indicate that today, more than 50 000 dams (Berga et al., 2006) involving nearly 50 percent of the world’s large rivers have been already constructed. Most of these dams are concentrated in species-rich areas and in sites that overlap with ecologically sensitive or critical habitats for conserving rare and threatened species. The Environmental Impact Assessments of the dams from around the world have brought about a radical realization that large dams that were thought to be overwhelmingly contributing to huge economic benefits are gradually eroding the natural base on which human societies thrive. Hydropower dam operations have been responsible for near-extinction of endangered species, flooding of forests and terrestrial wildlife habitats, causing irretrievable loss of carbon sinks, alteration of flows leading to significant loss of aquatic biodiversity and reduction in resource base that have resulted in decimation of subsistence economy and deprivation of livelihood opportunities. The challenges of combining economic benefits from hydropower development and protecting ecosystems goods and services that sustain societies and biological wealth on earth are therefore far greater than those associated with developments in other sectors. These challenges become further complicated when the cumulative impacts of multiple dams in a river basin become much greater than the simple sum of their direct impacts. A range of planning tools, for example, sectoral environmental assessments (EAs), basin-wide EAs, regional EAs, and cumulative EA can be used to innovate and improve impact assessment outcomes for aiding in hydropower development planning. This chapter puts forward the merits of conducting Cumulative Impact Assessment (CIA) of hydropower projects for identifying basin-wide impacts that may represent aggregate, multiple and altogether new impacts on ecological systems. Such an approach would be most appropriate for assessing risks for biodiversity elements and ecosystem services at the landscape levels so that energy planners can plan for dams in locations that are least disruptive to key ecological processes, and to operate them in ways that protect biodiversity and maintain key ecological services.