EMD F-units are a line of diesel-electric locomotives produced between November 1939 and November 1960 by General Motors Electro-Motive Division and General Motors-Diesel Division. Final assembly for all F-units was at the GM-EMD plant at La Grange, Illinois, and the GMDD plant in London, Ontario. They were sold to railroads throughout the United States, Canada and Mexico, and a few were exported to Saudi Arabia. The term F-unit refers to the model numbers given to each successive type, all of which began with the letter F. The F originally meant "fourteen", as in 1,400 horsepower (1,000 kW), not "freight". Longer EMD E-units for passenger service had twin 900-horsepower (670 kW) diesel engines. The E meant "eighteen" as in 1,800 horsepower (1,300 kW). Similarly, for early model EMD switchers, S meant "six hundred" and N meant "nine hundred horsepower".

A diesel locomotive is a type of railway locomotive in which the power source is a diesel engine. Several types of diesel locomotives have been developed, differing mainly in the means by which mechanical power is conveyed to the driving wheels. The most common are diesel-electric locomotives and diesel-hydraulic.