An analytical thermal model of the project Apollo Lunar Module and a digital computer routine for processing the thermal model are described. The thermal model simulates the environmental control system, the crew, and the vehicle structure. The computer routine utilizes a unique computation technique to minimize computer time requirements. Experimental data from thermal vacuum tests of the Lunar Module and results of the transient thermal analysis are compared. Thermal predictions for nominal missions are compared with the Apollo 9 and 11 flight data.