Variable rate application of nitrogen using canopy sensors requires integration of systems for sensing, data processing, and application rate control. In this presentation, we discuss these systems from an engineering perspective and compare alternatives. We review commercially available active optical sensors as well as other sensors that may provide auxiliary information useful to nitrogen rate determination. We discuss required data processing and integration steps, including alternative rate algorithms. We also review and compare various methods of application rate control that can be used for liquid and granular fertilizer. The overall goal is to show how sensors and variable-rate algorithms interact with application control hardware and software, and to illustrate what engineering considerations are important in the design and evaluation of an overall variable rate nitrogen system.