ATK is an American motorcycle and all-terrain vehicle company founded in 1985 and located in Centerville, Utah, USA. As of 2016, it has been operating primarily to support previously sold models through parts and service manual distribution. While ATK was initially founded on in-house chassis designs and modified sourced engines, the brand has primarily focused on acquisition and badge-engineered models from multiple companies worldwide since 2004.

ATK was founded by Horst Leitner, an Austrian-born engineer, Grand Prix motocross racer, and International Six Days Trial gold medallist, following his relocation to the United States in 1980. In 1985, after successfully patenting a new motorcycle drive-tensioning system and marketing custom-frame kit bikes designed to accept Honda XR350 engines, Leitner was approached by Puch, who offered to fund a motorcycle company based on his designs. Leitner named the new venture ATK after his patented device to eliminate chain torque for improved handling. Known later as the A-Trak, Leitner originally called this device the Anti-Tension Kettenantrieb (German: Anti-Tension Chain Drive). His first prototype machine would produce the following placements:

By 1987, due to the high regard for ATK's chassis, which already utilized a Rotax engine, Can-Am approached ATK about creating a replacement for their off-road line using dated 250 cc and 400 cc two-stroke Rotax engines. From this foundation, ATK created the 250 and 406 models. These motorcycles retained many of ATK's unique chassis features, including a countershaft-mounted rear disc brake, side-mounted single rear shock with no linkage, offset fuel tank filler, air filter beneath the gas tank, and backward-facing brake pedal.

As the prototypes were delivered, Can-Am cancelled the deal and announced their withdrawal from the off-road motorcycle market. This led to an agreement between Leitner and North American Can-Am dealers to fund the production of the 406 and future motorcycles. ATK gained a nationwide dealer network and funding, and Can-Am dealers retained a product to sell. From 1989 until 1995, ATK was the fifth largest off-road motorcycle manufacturer in North America. An article by Motocross Action magazine in December 2019 states that ATK produced thousands of four-stroke motorcycles since 1985, predating Japanese manufacturers' move to four-stroke engines by over a decade.

ATK initially released the 560, based on the displacement of its 562 cc (34.3 cu in) Rotax four-stroke engine. Modifications to the frame would see the designation change to the 604, which was used until the motorcycle received an engine revision in 1994. In 1988, ATK released the 406, the two-stroke air-cooled Rotax-powered model that would become synonymous with their two-stroke line. The two-stroke line was also the first to receive a second bike, a 250 cc (15 cu in) counterpart.

In 1991, Leitner sold ATK to an investment company, who would move the corporation to Utah. The new owners would expand the product line-up greatly, offering a second four-stroke, the 350, as well as electric-start and lightened variations of most models. The company also released ATK's first ATV, based on the 604, and XR kits, a full set of ATK accoutrements that accepted a Honda XR350 engine.^{[clarification needed]}

1994 would see the 604 revised into the 605, as well as the introduction of the 600 Flat Track model. 1996 brought the liquid-cooled 250 and 260 lines, as well as a Limited Edition 406 send-off model as ATK retired their open-class two-stroke offering. 1999 would see the introduction of the 50MX, an introductory and 125 2 stroke. motocross/pit bike model sold for two years. In 2003, with the purchase of Cannondale Motorsports, ATK began to assimilate the acquired models into their line-up. Thus began a new era for the company, as it began to badge-engineer its products.

The acquisition of Cannondale Motorsports by ATK was the result of a failed $80-million dollar+ investment between Cannondale Bicycle Corporation and private equity firms in the late 1990s to create a subsidiary producing off-road motorcycles and ATV vehicles. First announced at the 1998 Cincinnati Motorcycle Dealer Show (at which Cannondale began accepting dealer orders), their X400 Motocross bike would not appear on showroom floors until 2001. The delays in delivery were the result of several engineering issues related to the many advancements Cannondale attempted to incorporate into the motorcycle, including two engine redesigns. The potential of the new machine was found so impressive by Dirt Bike Magazine, they named it Bike of the Year, two years ahead of its release. In February 1999, Dirt Bike editor Ron Lawson was quoted regarding the unreleased bike's possible appeal to "older moto guys who want kind of a status symbol" as opposed to novice motorcyclists.