"Continuum wave theory predicts forward and reverse combustion wave thichness on the order of a few cm, at most. It is argued that coal, especually subbituminous varieties which tend to shrink upon heating, probably does not satisfying the fine scale porosity assumption underlying continuum wave models. Instead combustion and other thermal attack along expanding fissures must play important roles in both forward and reverse combustion. For forward combustion aimed at gas production by carbon-steam reaction, mechanical degeneration appears also to be important. The status of modeling reverse combustion along coal fisures is reviewed in some depth. There is evidence that such propagation is highly sensitive to irregularities in the structural state of the coal. Under some circumstances radiative heat transfer can control the process. Exploited region development is classified into processes at non-permeative and permeative fire faces. At permeative fire faces forward combustion along fissures can enhance burnout and rubbelization. The paper is directed mainly at shrinking coals but problems with swelling coals are breifly discussed."