River Ganga has been the source of physical and spiritual sustenance of Indian civilization for millennia. Counted among the major riverine wonders of the world, it is a mega diverse ecosystem that is home to a wide variety of relic species from the tiny and microscopic to macroscopic forms ─ from planktonic to benthic and free floating to submerged macrophytes. It is a hierarchical system of more than 2,000 biological forms, out of which more than 1,600 are the forms at 1st, 2nd and 3rd trophic levels. This book, based on extensive ecological analyses carried out for the preparation of the Ganga River Basin Management Plan – 2015 by a 7-IIT Consortium, is the first systematic compilation of major biological forms (phytoplankton, zooplankton, periphyton, zoobenthos, ichthyo-fauna and higher vertebrates) in river Ganga and her major tributaries, largely based on information culled from scattered documents. Spatial and temporal changes in ichthyofaunal diversity in terms of catch and composition is also presented briefly.