This chapter presents an overview of proximal sensing techniques used in practical precision agriculture, with the overall aim of contributing to more sustainable crop production. To provide perspective on the usefulness of crop sensors, the chapter starts with an example of why site-specific crop management is important and how it relates to sustainable production. The basics of crop canopy reflectance are then presented, followed by an introduction to two practical precision agriculture techniques, site-specific weed management and real-time crop sensing, mainly focusing on within-field nitrogen application. Two case studies in which different types of proximal sensors are used, individually or in combination with other techniques (UAVs and satellites), are described. Finally, potential future developments are briefly discussed.