This paper is based on a Simon W. Freese lecture given at the ASCE North American Water and Environment Congress '96, Anaheim, Calif., on June 24, 1996. The role of water quality modeling in providing input to the decision-making process through understanding, dialogue, and consensus is discussed. The evolution of models is seen in three stages. During the first stage (1925 to about 1980), all sources (point, nonpoint, and sediment) were external to the model, but only point sources were directly linked to the originating input. During the second stage (about 1980 to 1990), sediment models were coupled to the water column and hydrodynamic and watershed models were linked. A link was then established from watershed models to the input of the watershed. During the third stage (currently under way), airshed models are being incorporated with expansion to include other aspects of aquatic ecosystem. The Chesapeake Bay is used as an illustration. Issues of model credibility and confirmation are discussed; ultim...