Explosive compaction of spent shale in an in situ retort has been suggested as a means of obtaining the void volume required for the successful generation of subsequent retorts. Static compaction experiments and explicit finite difference calculations have been used to provide a preliminary evaluation of the spent shale compaction concept. These experiments have provided basic compaction data on fragmented and reconstituted samples of oil shale from the Tipton Member of the Green River Formation in Wyoming. Data were obtained on samples initially at room temperature and on samples heated to retort temperatures as high as 600 C. The initial fracture void volume, before retorting, varied from 25 percent to 50 percent. The data demonstrate that the energy required for compaction decreases substantially as the retorting temperature increases. The grade of oil shale was also a factor, as richer shales appear to offer less resistance to compaction for the same retorting history. One-dimensional finite-difference calculations, representative of simplified field geometries, were used to evaluate the sensitivity of compaction behavior to explosive characteristics, seismic coupling and compacting properties. These calculations indicate that for compaction at elevated temperatures, no more than a few pounds of explosive per ton should be sufficient to provide adequate compaction for a continuous in situ process. That is, all the void volume required for a new retort module may be economically obtained by explosively compacting the adjacent spent module. Further studies, including dynamic compaction experiments and two-dimensional finite-difference calculations, are required to refine the design of specific in situ operations.