The aims of modern fuel technology are those of attaining the most efficient production and utilization of solid, liquid, and gaseous fuels. However, during the past decade and more a considerable inefficiency in exploitation of these raw materials has been experienced by using "ordinary" mining or recovery methods. In many cases, original high-grade occurrences have been depleted and the materials of interior value necessarily have been neglected and, thus, numerous deposits have been declared exhausted when mining and recovery processes have become uneconomical or impracticable. Therefore, the objectives of continued research in the field of fuel technology must be devoted largely to improvements of recovery methods and to the resultant more complete exploitation of national fuel resources.