Eutrophication has become a global problem that is likely to intensify in coming decades because of increase in human population,land conversion in to layouts, urbanized catchment area and diversion of the inflow. One of the main difficulties in studying loss of biodiversity due to eutrophication is the absence of previous records of species composition. Zooplankton is an important component in aquatic ecosystems, their community structure is shaped primarily by the physical and chemical environment and biological parameters, and these communities have potential value as indicators of changing trophic condition. This book is usefull for the young researchers as it guides how to characterize a water body and how to assess the trophic status by using indicator zooplankton. It also give baseline data of some South Indian reservoirs for further research.