To develop and direct a water quality program for estuaries and tidal inlets, a model is required to predict the mixing and movement of wastes released into the water. Many different water quality models have been developed and used to fulfill this need. The available models will be examined in terms of the representations and simplifications used, and the range of conditions under which each might be satisfactorily employed will be indicated. Only models of miscible neutrally buoyant wastes are considered. The review is in two parts. A scheme for classifying models is introduced herein and its application in selecting the appropriate model to meet the needs of a user is explained. A second paper scheduled for publication later will review the different models that have been used to predict water and waste movement in tidal waters, using the classification scheme developed herein as a basis.