Computer modeling is becoming an integral part of decision making in water pollution control. Problems increasingly involve complex interactions among elements of the environment and large, multi-media modeling systems must be built to understand these interactions. Expert systems is an innovative methodology that can assist in building, using, and interpreting the output of these models. The paper reviews the use and evaluates the potential of expert systems technology in environmental modeling and describes elements of an expert advisor for the stream water quality model QUAL2E. QUAL2E has a long history of use both in the United States and worldwide and is a proven, effective modeling tool for analyzing the dissolved oxygen balance in a stream or river. Because of the widespread usage, a body of experience and empirical knowledge about the computer program has been gained that is ideal for codification in an expert system.