Harley-Davidson Motor Company (nyse: HOG, formerly HDI) is an American manufacturer of motorcycles based in Milwaukee, Wisconsin. The company sells heavyweight (over 750 cc) motorcycles designed for cruising on the highway. Harley-Davidson motorcycles (popularly known as "Harleys") have a distinctive design and exhaust note. They are especially noted for the tradition of heavy customization that gave rise to the chopper-style of motorcycle.

Harley-Davidson attracts a loyal brand community, with licensing of the Harley-Davidson logo accounting for almost 5% of the company's net revenue ($41 million in 2004). Harley-Davidson supplies many American police forces with their motorcycle fleets.

In 2003, the Buell Motorcycle Company became a wholly-owned subsidiary of Harley-Davidson, the same year that Harley-Davidson celebrated its 100th birthday. In August 2008, Harley-Davidson purchased the Italian motorcycle manufacturer MV Agusta.

The beginning

In 1901, William S. Harley, age 21, drew up plans for a small engine with a displacement of and four-inch (102 mm) flywheels. The engine was designed for use in a regular pedal-bicycle frame.

Over the next two years Harley and his childhood friend Arthur Davidson labored on their motor-bicycle using the northside machine shop at the home of their friend, Henry Melk. It was finished in 1903 with the help of Arthur's brother, Walter Davidson. Upon completion the boys found their power-cycle unable to conquer Milwaukee's modest hills without pedal assistance. Will Harley and the Davidsons quickly wrote off their first motor-bicycle as a valuable learning experiment.

Work immediately began on a new and improved second-generation machine. This first "real" Harley-Davidson motorcycle had a bigger engine of with flywheels weighing . The machine's advanced loop-frame pattern was similar to the 1903 Milwaukee Merkel motorcycle (designed by Joseph Merkel, later of Flying Merkel fame.) The bigger engine and loop-frame design took it out of the motorized-bicycle category and would help define what a modern motorcycle should contain in the years to come. The boys also received help with their bigger engine from outboard motor pioneer Ole Evinrude, who was then building gas engines of his own design for automotive use on Milwaukee's Lake Street.

The prototype of the new loop-frame Harley-Davidson was assembled in a 10- by 15-foot (3 by 5 meter) shed in the Davidson family backyard. Most of the major parts, however, were made elsewhere, including some probably fabricated at the West Milwaukee railshops where oldest brother William A. Davidson was then toolroom foreman. This prototype machine was functional by 8 September 1904 when it competed in a Milwaukee motorcycle race held at State Fair Park. It was ridden by Edward Hildebrand and placed fourth. This is the first documented appearance of a Harley-Davidson motorcycle in the historical record.