The purpose of this research project was to provide much needed information for the development of active canopy sensors as a N fertilizer management decision tool in corn production. Active sensors can be used as an information source regarding plant N status and, can be calibrated to help adjust in-season N applications when corn N deficiencies exist. The adoption of precision farming technology, such as active sensors, is growing in Iowa and much of the U.S. Corn Belt. This Ph.D. dissertation is presented as a series of five chapters. The first chapter is an overview of the dissertation research. The second chapter is entitled "Factors affecting active canopy sensor performance and reflectance measurements". The third chapter is entitled "Using active canopy sensors to quantify corn nitrogen stress and nitrogen application rate". This chapter was published in Agronomy Journal, Volume 102, Issue 3, 2010, pages 964-971. The fourth chapter is entitled "Active canopy sensing to adjust nitrogen application rate in corn". The fifth and final chapter provides the overall conclusions of the dissertation research. Chapters two and four are manuscripts that are intended to be submitted for publication in a journal from the American Society of Agronomy.