The world population is increasing rapidly, and by 2050, it is estimated that there will be nearly nine billion people to feed (Cohen, 2003). Agricultural production to feed this large population will be severely constrained by a lack of additional arable land combined with a diminishing supply of water and increasing pressure to protect the quality of water resources beyond the edge of agricultural fields. These constraints mean that it will be increasingly imperative to prevent losses in crop productivity due to water stress, nutrient deficiencies, weeds, insects, and crop diseases. Those losses in productivity often occur at specific locations within fields and at critical growth stages. They are not typically uniform in severity across locations within a field. Thus, farmers must take measures to identify where crop stress occurs in a timely fashion, they must identify what is causing crop stress, and they must try to use management practices that overcome crop stress at specific locations and times.