This chapter describes the geologic framework of regional groundwater flow in the Willamette Valley, a conceptual model of the regional groundwater flow system, the results of cross-sectional numerical simulation of groundwater flow, and discusses implications for surface-water quality. This work was part of a comprehensive hydrogeologic investigation of the Puget-Willamette Lowland in Oregon, Washington, and British Columbia, Canada, conducted as part of the US Geological Survey Regional Aquifer-System Analysis program. The Willamette River drainage basin lies between the crests of the Cascade and Coast Ranges in western Oregon, encompassing approximately 11,500 mi². The particle-tracking program calculates and records the paths of hypothetical particles of water as they move through the simulated system. The main source of groundwater recharge to the regional flow system is precipitation onto the valley floor and the Columbia River Basalt uplands in and around the valley. Most groundwater flow occurs in the Willamette aquifer and the overlying Willamette Silt.