Arc Hydro is a state-of-the-art geospatial RDBMS technology for integrating hydrography and hydrology with GIS. Arc Hydro is a geospatial and temporal data model for water resources designed to operate within ArcGIS. A data model is distinct from a simulation model in that a data model provides a standardized framework for storing information, but contains no routines to simulate hydrologic processes. The data model is typically coupled with one or more simulation models with data and information being transferred from Arc Hydro to a model and results being returned to Arc Hydro. Arc Hydro, therefore, provides a means for linking simulation models through a common data storage system. Arc Hydro Tools is a set of ArcGIS tools that “exercises” the Arc Hydro model. It populates various fields within an Arc Hydro geodatabase for a river basin starting from a digital elevation model (DEM). This paper will describe the Arc Hydro data model and Arc Hydro tools for ArcGIS. A case study will be presented demonstrating the application of Arc Hydro tools for modeling a watershed located in Pennsylvania. The lessons learned will be discussed. Typical problems and challenges in using the Arc Hydro tools will be discussed and solutions and workarounds will be presented.