"Shale oil production by thermal decomposition or the organic material in the shale has been described as a pseudo first order process. In a practical process the shale is heated to the maximum temperature over a finite period or time. The products or retorting remain in a heated zone for varying periods of time. The nature of the product depends on both the temperature range over which thermal decomposition takes place and on the residence time of the products in a hot zone. Temperature measurements in a retort are not readily related to the product even if coupled with flow rates through the retort. This chapter develops a parameter that combines the elements or temperature and residence time into a single number that can be used to indicate instantaneous retorting conditions or to compare various thermal processing techniques for oil shale. The parameter developed for comparison or control or the retorting process has the dimension of temperature. It could be defined as an ""effective temperature"" or a ""Retorting Index."" In the laboratory, this index was developed by relating the ethylene/ethane ratio in the product gases from retorting or shale to a measured temperature in the retorting zone. Residence times of the products in the hot zone were nearly constant. In a commercial-size retort , residence times could vary over wide ranges. Residence time variation causes a change in the observed ratio. The calculated parameter also changes and so reflects the changing retorting conditions. Application of the parameter to Fischer assay retorts, gas combustion retorts, and to entrained solids retorts is illustrated with examples. The easily calculable Retorting Index is applicable to in situ retorting as well as those retorts used as examples."