Today’s farmers are increasingly reliant on sensors for their farming operations. This chapter addresses the use of proximal sensors to evaluate a crop during the growing season. After briefly discussing the importance of sensors in modern farming and the crop properties which are of interest, we review the evolution of crop sensors. We then discuss three issues that are currently faced in sensor development: the limitations of current crop canopy sensors; detecting stresses other than nitrogen and sensor fusion and high-throughput phenotyping. Three case studies are presented, and future trends in research are considered.