Groundwater models, when properly constructed and calibrated, can be very effective tools for understanding groundwater flow and contaminant transport through complex subsurface environments. However, modeling in general has garnered a somewhat tarnished reputation due to a history of significant discrepancies between simulated and observed contaminant travel times. This is particularly true in complex hydrogeologic settings such as karst and fractured rock aquifers where heterogeneities dominate. Recent advances in computing power, modeling software, and most importantly, the understanding of how to design models in complex settings have dramatically improved this situation. This specialized modeling session provides an overview of these advances through presentations by modelers that have developed effective simulations of complex hydrogeologic problems and summarize the most important aspects of successful modeling projects.