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The concept of total maximum daily loads (TMDLs) to achieve numeric water quality criteria for nonpoint sources is similar to that of waste load allocations that have successfully been used by states for many years to establish point-source effluent limitations. Yet the inherent differences between point and nonpoint sources of contaminants and associated transport mechanisms raise significant concerns and technical challenges in translating this concept to nonpoint sources, for application in a regulatory context. Some states, including Iowa, are utilizing TMDLs to accomplish comprehensive watershed assessment to target the implementation of voluntary conservation programs to specific water quality goals. Prescriptive models can be used in these assessments to predict nonpoint source contaminant transport and impact of various best management practices/systems. Many models are available for use and will give somewhat differing results. A study in Iowa is being planned to provide technical guidance on the selection and use of models that best fit the Iowa landscape, as well as facilitate input to the selection process by affected economic and environmental interests. Relative to nutrient criteria, the U.S. EPA is in the process of helping States and Tribes develop regionally-based numeric values for rivers and streams and lakes and reservoirs (as well as for estuarine and coastal marine areas and wetlands). Nutrient overenrichment is a major source of water pollution in the U.S., resulting in euthrophication. Region-specific nutrient criteria are expected to help address this pollution problem, and criteria are expected to protect and support the designated uses of a water body. In agricultural areas where drastic changes in land use and drainage/hydrology have occurred, there is concern for how these criteria will be set and used in the TMDL process. Comparisons made between possible criteria and existing conditions, and the potential to reach these criteria with reasonable implementation of BMPs, raises the issue of attainability.