The following report is a description of a 7 year effort to develop a theoretical understanding of the underground coal gasification process. The approach used is one of the mathematical model development from known chemical and principles, simplification of the models to isolate important effects, and through validation of models to isolate important effects, and through validation of models with laboratory experiments and field test data. Chapter I contains only introductory material. Chapter II describes the development of two models for reverse combustion: a combustion model and a linearized model for combustion front instability. Both models are required for realistic field predictions. Chapter III contains a discussion of a successful forward gasification model. Chapter IV discusses the spalling-enhanced-drying model is applicable to prediction of cavity growth and subsidence. Chapter VI decribes the correct use of energy and material balances for the analysis of UCG field test data. Chapter VII shows how laboratory experiments were used to validate the models for reverse combustion and forward gasification. It is also shown that laboratory combustion tube experiments can be used to simulate gas compositions expected from field tests. Finally, Chapter VII presents results from a comprehensive economic analysis of UCG involving 1296 separate cases. 37 refs., 49 figs., 12 tabs.