The challenge facing the oil shale industry is to devise a simple and efficient retorting process that operates at low cost. This will require a thorough understanding of the important physical and chemical processes that occur during retorting, mathematical modeling to analyze the complex interactions, and sophisticated engineering to design a retorting process having the minimum cost. Retorting processes can be classified into two groups, one using hot gas as the heat transfer medium and one using hot solid material. We have constructed mathematical models of two generic processes from each of these groups to represent state-of-the-art processes as well as those under development. The models are based on detailed reaction kinetics, thermodynamics, and process physics. Using these models, we have calculated the process characteristics and the complete mass and energy balances for 50,000-barrel/day shale oil plants. We have made comparisons of these four processes to illustrate their advantages and disadvantages. The comparisons include retort vessel volume, heat exchange capacity, power requirements, water requirements, and waste gas volume. The results indicate no present easy choice of a best process. Oil shale process technology is neither highly developed nor mature, and substantial improvements can be expected from further research and development, and from industrial experience. The hot gas processes can be viewed as first-generation technology, and the hot solid processes, if they can be successfully developed, may be second-generation technology.