Since 1999, the National Energy Technology Laboratory has conducted helicopter frequency domain electromagnetic (FDEM) surveys of 30 sites to identify hydrologic problems that have resulted from mining activity. The FDEM data from these surveys was processed to generate conductivity maps and conductivity/depth images (CDI), which together show the lateral and vertical distribution of ground conductivity. This information has been used to determine: 1) the location, depth, and thickness of regional water tables and perched water tables, 2) the location of infiltration zones, 3) the location of pyritic wastes either on the surface or at depth, 4) the location of flooded mine workings (if filled with conductive water and located at depths less than 50 m), and 5) the likely locations for springs or mine discharges. Helicopter FDEM results have been validated using down-hole geophysical measurements, traditional hydrologic measurements from dense networks of groundwater monitoring wells, ground-based electromagnetic surveys, airborne thermal infrared imagery, and field reconnaissance. Helicopter FDEM surveys cannot be used in heavily populated areas or areas near power lines. The exploration depth will be reduced in areas with conductive cover such as contaminated surface impoundments or clay layers. Successful and unsuccessful FDEM surveys will be discussed.