A review of the total field testing effort to date in the Department of Energy Underground Coal Conversion program is presented. Since its inception in 1972, this program was validated and significantly extended the state of the art that was primarily available from pre-1965 Soviet publications. In general, the results of the field tests have been very positive, and indicate strongly that UCC could be used commercially in this country before 1990. Some problems have been encountered with linking techniques and override situations further work on these problems is required. A distinguishing feature of the DOE program has been the development and use of sophisticated diagnostic instrumentation, which has led to a much more quantitative understanding of the process than was previously possible. Further efforts in this area are also planned.