The first version or ''base model''is described of a mathematical model of vertical, forward combustion retorting of oil shale that was developed through 1974 at the Laramie Energy Technology Center in Laramie, Wyoming. The model was written to simulate conditions and processes occurring in LETC's experimental batch-type 150-ton retort. Information presented deals with the reasoning and justification for developing the model, a description of the modeling effort, and a comparison of the yields, retorting rates, and other information obtained from the model as compared with actual results from the 150-ton retort. Good qualitative agreement between the ''base model''and actual experimental results is found. However, quantitative agreement is not realized in this version of the model; some reasons for this are discussed. Current and future work on the model is also discussed.