Underground coal gasification field test results obtained since 1976 are reviewed, illustrating the important role that the UCG process feature of a varying reactor geometry has on resource recovery and gas quality. The different instrumentation used on these tests is then reviewed, particularly as to its effectiveness or lack of in defining process geometry. Instrumentation such as thermocouples, HFEM, acoustic and surface resistivity are discussed with respect to concept, cost, resolution, data acquisition and data analysis. Results indicate that when instrumentation is appropriately deployed it can provide significant insight into the dynamics of reactor growth.