DIESEL ENGINES have proven to be the most efficient power source available today. To retain this superiority will require further improvements in specific engine output and reduced fuel consumption while meeting future emission requirements with high levels of reliability and durability. Engines will have varying design trends depending on horsepower range, emission requirements, market needs and available fuels. New advanced engine components will be needed. Spark-ignited and dual fuel versions of diesel engines will gain more acceptance. Alternate fuels will be used as petroleum fuel costs rise. The effect of these changing requirements on future diesel engines for the 1990's will be reviewed in this paper.