For many corn production fields, variable-rate N applications can optimize production and minimize off-field N losses.  Variability in N fertilizer need is a function of both year-to-year climate differences (e.g., precipitation and temperature) and point-to-point soil differences (e.g., mineralization potential and denitrification potential).  Crop canopy reflectance sensors employing artificial light sources have allowed researchers and producers to explore procedures accounting for this variability in N fertilizer need and, when employed on a variable-rate applicator, automatically adjusting N application rates. This procedure is based on the fact that corn needing N is lighter in color, usually smaller, and reflects light differently than corn that has sufficient N.  This work reports on using these sensors in quantifying crop N needs and the potential subsequent environmental benefits.  As an example, a 2004-05 study conducted on 12 producer fields in Missouri found that economic optimal N rate (EONR) was significantly related to readings from the sensors.  Averaged over all fields, sensor-based applications reduced N fertilizer rate from what the producer applied by 37 kg N/ha.  As N-rate approached EONR, both yield efficiency and N fertilizer recovery efficiency declined, while post-harvest inorganic N levels increased. The potential reduction in N lost from corn production fields into rivers and streams by using this sensor technology will be examined for the case of the U.S. Midwest.  Adoption of sensor-based N management will occur once profitability is demonstrated and environmental benefits are documented.