The Department of Energy Atmospheric Radiation Measurement (ARM) program is a climate research user facility operating stationary ground sites in three important climatic regimes that provide long-term measurements of climate-relevant properties. ARM also operates mobile ground- and ship-based facilities to conduct shorter field campaigns to investigate understudied climate regimes around the globe. Airborne observations by ARM's Aerial Facility (AAF) enhance the surface-based ARM measurements by providing vertical and horizontal context for surface based measurements, validation of remote sensing measurements made from space or the surface, and information which is not accessible by surface- or space-based remote sensing methods. The separately funded ARM Unmanned Aerospace Vehicle (UAV) program has carried out 12 missions between 1993 and 2006 relying on UAVs and piloted aircraft.