For over 30 years the Laramie Energy Technology Center has been actively engaged in oil shale technology research specifically to study various scenarios of oil shale conversion to some type of useable liquid product. One technique which appeared to have significant economic and environmental advantage over others was in situ retorting. Whether "true" or what later evolved and was known as "modified" in situ retorting would apply depended on a number of factors. In either case, successful recovery of substantial quantities of oil from a fractured mass of oil shale depended largely upon permeability, particle size distribution, the degree of control that could be maintained in directing the retorting front and the uniformity of the rate of advance of the front.