Groundwater modeling is undergoing a revolution driven by the need to incorporate uncertainty in the conceptual model, numerical model, and in model calibration. Additionally, new GIS packages and new ways of modeling (e.g., with unstructured grids) provide powerful tools to improve how we model. In this session, we will explore some of these exciting new developments in groundwater modeling. In the past, groundwater modelers focused on simulating a single version of reality. However, modelers now acknowledge that unavoidable gaps in hydrogeological field data mean that there is uncertainty in the conceptual model that underlies every numerical groundwater model. Moreover, modelers are gradually accepting that inverse (parameter estimation) codes, which incorporate uncertainty analysis as part of the calibration, provide a better way to calibrate models. Furthermore, new GIS tools help in formulating the conceptual model while unstructured grids allow much needed flexibility in constructing the numerical model. New codes are available for complex problems involving saltwater intrusion in coastal aquifers and advances are being made in representing heterogeneities in subsurface properties, which is critical in delineating contaminant pathways. Ongoing research is also yielding a suite of codes for analyzing transport of contaminants with efforts toward aquifer remediation.