The obvious application for active crop canopy sensors is to assess the need to make in-season nitrogen (N) applications to crops like wheat, corn, sorghum, rice, cotton, and sugarcane. These crops all require large amounts of N fertilizer to optimize yields. Spatial variability in soil properties is a given and precipitation amount and timing are uncertain, so it follows that soil N availability is both spatial and temporal. It is frequently assumed that plant chlorophyll status and biomass production are a function of crop N status, but this linkage can be confounded by cultural practices, soil-rooting conditions, water availability, and deficiencies in other nutrients. Active sensors not only characterize relative crop vigor at the field scale, but can also provide other types of potentially meaningful information depending on sensor design and specifications. For example, the size and shape of a sensor’s foot-print influences the ability to extract information about plant-to-plant uniformity. Individual waveband information can be used to make inferences about plant height and canopy density. Selection of specific wavebands and associated vegetation indices can be used to make management decisions related to irrigation and harvesting. Row-to-row variability in sensor readings are frequently related to soil compaction patterns. Inconsistencies between soil-based management zone delineation and active sensor data should not be alarming considering the temporal and spatial nature of the factors affecting plant growth. It is noteworthy that temporal data provided by sensors can be a powerful scouting tool to further examine spatial soil and crop growth factors.