High-boiling shale oil produced from Rocky Mountain oil shale can be reduced in molecular weight by recycle thermal cracking and by coking. Selected research on the production of diesel fuels from shale oil is reviewed. Diesel fuels of good quality have been made from cracked shale oil by acid and caustic treating. Diesel oil made by this process performed acceptably in an in-service test for powering a railroad engine in a 750-hour test. Better quality diesel fuels were made by hydrogenation of a coker distillate. Even better quality diesel fuels, suitable also for use as high-quality distillate burner fuels, have been made by hydrocracking of a crude shale oil from underground in-situ retorting experiments.