Publisher Summary This chapter focuses on the linkage between subsurface tile drainage of agricultural lands and nitrate in surface waters, and the effect of uncontrollable factors and controllable factors on nitrate losses to subsurface drainage. Subsurface drainage is a common water management practice in highly productive agricultural areas with poorly drained soils that have seasonally perched water tables or shallow groundwater. This management practice increases crop productivity, reduces risk, and improves economic returns to crop producers. Agricultural drainage water has been identified as a major contributor to the nitrate-N loading of receiving waters. Research conducted at widely different scales of watershed basins point to the fact that agricultural systems do affect nitrate levels in river waters. Long-term, subsurface drainage research, which integrates the effects of climatic variability, soil properties, and various cropping systems, is vital to understanding of nitrate losses to subsurface drainage.