The long-standing conceptual model of groundwater fl ow, built on Darcian averaging over representative elementary volumes, provides the groundwater development industry with unquestionable predictive value. Conversely, the advective-dispersive conceptual model of contaminant and reagent mass transport, built on the averaging Darcian framework, has not provided such a reliable representation of mass transport processes for the scales at which we track contaminant migration and apply remedial processes. In contaminant and reagent mass transport, the distinctions between Darcian hydrogeology and the heterogeneous domain of remediation hydraulics become clearly evident and critically important. As suggested 40 years ago by Professor Theis, heterogeneities in aquifer structure play a dominant role in the migration and storage of solutes in aquifers and to obtain acceptable predictive power, our conceptual model of contaminant and mass transport must account for heterogeneous aquifer structure. In this chapter, we examine physical and chemical mechanisms that govern solute migration between fl owing and essentially static groundwater, the mobile and immobile porosity. Although we can describe the distributive mechanisms and build conceptual site models that are sensitive to aquifer heterogeneities, there is no way to escape the requirement that we map aquifer structure as a foundation for understanding contaminant distribution patterns at each site.