Recent field tests of underground coal gasification in the United States have led in a greater degree of optimism for the eventual commercialization of this technique due in part to a more complete understanding of the process itself. The most recent test at Hanna, Wyoming, has demonstrated that a high quality low Btu gas can be predictably produced for ling periods and that process control techniques can be utilized. Furthermore, with proper site selection the performance of the underground reaction can equal if not exceed that of an air blown surface gasification system. Recent tests have demonstrated cold gas efficiencies of 85 percent and energy return ratio of 5:1. Most important to an efficient process is an explanation of this high performance. Results show that the key determinant of high efficiency is the inherent nature of thick seams of coal which shrink and become friable when heated. These properties allow for a presence which provides a self-propagating mechanism for a packed bed to achieve the intimate contact between the reactants necessary for optimum gasification conditions.