Electro-Motive Diesel (abbreviated EMD) is a brand of diesel-electric locomotives, locomotive products and diesel engines for the rail industry. Formerly a division of General Motors, EMD has been owned by Progress Rail since 2010. Electro-Motive Diesel traces its roots to the Electro-Motive Engineering Corporation, founded in 1922 and purchased by General Motors in 1930. After purchase by GM, the company was known as GM's Electro-Motive Division. In 2005, GM sold EMD to Greenbriar Equity Group and Berkshire Partners, and in 2010, EMD was sold to Progress Rail, a subsidiary of the heavy equipment manufacturer Caterpillar. Upon the 2005 sale, the company was renamed to Electro-Motive Diesel.

EMD's headquarters and engineering facilities are based in McCook, Illinois, while its final locomotive assembly line is located in Muncie, Indiana. EMD also operates a traction motor maintenance, rebuild, and overhaul facility in San Luis Potosí, Mexico.

As of 2008, EMD employed approximately 3,260 people, and in 2010 it held only 30% of the market for diesel-electric locomotives in North America. Their only significant competitor is Wabtec-owned GE Transportation, which holds the remaining 70% market share of the North American market.

Harold L. Hamilton and Paul Turner founded the Electro-Motive Engineering Corporation in Cleveland, Ohio, in 1922, soon renaming it to Electro-Motive Company (EMC). The company developed and marketed self-propelled railcars using General Electric's newly developed internal combustion-electric propulsion and control systems. Hamilton started his railroading career as a fireman, then locomotive engineer, on the Southern Pacific Railroad, then became a manager with the Florida East Coast Railway before he left railroading for a marketing position with the White Motor Company, an early manufacturer of trucks and buses, in Denver. Training and service agreements were part of White's marketing package that Hamilton would carry over to EMC. Aware of the needs of branch line services of railroads and the opportunities provided with GE's new internal combustion-electric propulsion and control technology, he quit his position with White and set up shop in a Chicago hotel with his partner and a designer to develop and market a new generation of self-propelled railcars. In 1923 EMC sold two gasoline-powered rail motor cars, one to the Chicago Great Western and the other to the Northern Pacific. EMC subcontracted the body construction to St Louis Car Company, electrical components to General Electric, and the prime mover to the Winton Engine Company of Cleveland, Ohio. The motorcars were delivered in 1924 and worked well, fortunate for the fledgling company, because the sales were conditional on satisfactory performance. In 1925 EMC entered full-scale production, selling 27 railcars.

In 1930, General Motors (GM) was seeking to enter production of diesel engines and broaden their range of applications. They purchased the Winton Engine Company, who had in their product line a variety of stationary and marine diesel engines and spark-ignition engines for heavy vehicles. GM saw EMC's role in developing and marketing Winton-engined heavy vehicles as fitting their objectives and purchased the company shortly after the Winton acquisition, renaming it Electro-Motive Corporation (EMC), a subsidiary of GM. Supported by the GM Research Division headed by Charles F. Kettering, GM's Winton Engine Corporation focused on developing diesel engines with improved power-to-weight ratios and output flexibility suitable for mobile use. Eugene W. Kettering, son of Charles Kettering, led Winton's side of the development project.

In 1933, EMC designed the power setups for the Zephyr and M-10000 streamliners, a breakthrough in the power and speed available with their propulsion systems. The Zephyr used the first major product of the new GM-Winton venture, a 600 hp, eight cylinder version of the Winton 201A Roots blown, uniflow scavenged, unit injected, 2-stroke diesel engine. As the Budd and Pullman Standard companies entered contracts to build more diesel-powered streamliners, they became major customers for EMC. Diesel power had been shown suitable for small, lightweight, high speed trains, in addition to its more established role in yard service.

Seeing opportunities to broaden the role of diesel in railroading, EMC invested in a new locomotive factory and started development work on the locomotives that it would produce. The factory headquarters on 55th Street in McCook, Illinois, west of Chicago, remains the corporate headquarters. The 1935 EMC 1800 hp B-B development design locomotives featured the multiple-unit control systems that became the basis of cab/booster locomotive sets, and the twin engine format that would be adopted for the newest Zephyr power units in 1936 and EMC's E series streamlined passenger locomotives that their new factory began producing in 1937. Prior to their introduction of the E units EMC was in production of switch engines, which remained the mainstay of their production until dieselization of freight and passenger service hit full stride in the mid-1940s.

The GM-Winton research and development effort continued through the mid-1930s, building on experience with the Winton 201A, to develop diesel engines to better meet the specific needs of locomotive use. The fruit of that effort was GM's new 567 engine, introduced by their renamed Cleveland Diesel Engine Division in 1938. The new engine upgraded the horsepower of EMC's E series locomotives to 2000 per locomotive unit and increased reliability substantially. Also in 1938, EMC increased its reach up the chain of locomotive production by transitioning from General Electric equipment to in-house produced generators and traction motors. With Eugene Kettering moving to EMC that year, EMC moved into a leading role in further development of GM's locomotive engines.