Water is an important element for life on the earth. It is an essential natural resource for environmental sustenance. In India, water quality modeling studies are carried out from fresh water to marine water ecosystems. Some of the examples are Tehri reservoir, Chilka lake, Oatcake at Kashmir, Kodaikanal lake, Ooty lake at Tamil Nadu, rivers like Ganges, Narmada, Kaveri, and coastal regions like Hoogly estuary, Paradip, Vishakhapatnam, Kakinada, Chennai, Mangalore coast, Konkan coast and Gujarat coast. Water quality modeling plays a vital role in water quality studies. Numerical models are to be successfully calibrated and properly applied and it is to be improved our understanding of the complex interactions among different parameters such as temperature, biological oxygen demand, dissolved oxygen, salinity, and eutrophication in the fresh water and sea water environment.