The principal objective in writing this detailed user's manual is to provide both the estuarine modeling specialist and non-specialist alike with sufficient information with which to exercise the Galveston Bay Project (GBP) hydraulic or hydrodynamic model in Galveston Bay water quality management evaluations. Brief comments concerning user requirements for applying this model to other estuarine systems are also included but this aspect is not emphasized. Introductory remarks caution the reader/user of the advisability of reading this manual thoroughly, as well as supplementing this document with important information contained in other references before actually utilizing this hydraulic model and interpreting its computational results.