Recent field experiments have demonstrated that the yield from an in situ retort is largely controlled by the characteristics of the rubble bed produced during the bed preparation step. This realization has focused the need to develop a predictive capability for the fragmentation step in the process of constructing an in situ retort. In addition, it is important that criteria for the blast results be developed that are based on a thorough understanding of retort yield as a function of bed characteristics. The influence of permeability structure on retort yield must be clearly defined.