Nitrate is one of the most common groundwater contaminants worldwide, primarily caused by agriculture, leaking sewer systems, and septic systems. While some subsurface conditions naturally attenuate nitrate, the coarse-grained, oxygen-rich aquifers common in the Southwest favor its persistence. Thus we have a problem, and it is growing because the usual response has been to find a new water source rather than remediate the existing one. However, the tide is starting to change as fewer new water sources are available, impacts can no longer be ignored, and treatment technologies improve. These feature articles provide the details.