Unmanned Aerial Vehicles (UAV) have been developed and applied to support the practice of precision agriculture. Compared to piloted aircrafts, UAVs can focus on much smaller crop fields to perform the site-specific management with high precision. In crop production management, UAVs have been developed and used for chemical application and remote sensing. Agricultural application of fertilizers and chemicals is frequently needed at specific time and location for highly accurate site-specific management. Routine monitoring of crop plant health is often required with super-resolution for highly accurate site-specific management as well. UAVs are the technology to fulfill the goals of such crop management. This research will overview the development of UAVs in precision agriculture. The development of an ultra low volume (ULV) sprayer for a UAV helicopter will be described. The results of the field study will be analyzed and discussed. Also, two other UAVs, a helicopter and a fix-wing airplane, will be studied and evaluated for remote sensing over crop fields. The integration of the spray technology and the remote sensing technology on the UAV systems provides a great potential to identify crop stresses and hence spray crop production and protection materials at different rates over small crop fields to realize highly accurate site-specific crop production management.