The automotive aftermarket is the secondary market of the automotive industry, concerned with the manufacturing, remanufacturing, distribution, retailing, and installation of all vehicle parts, chemicals, equipment, and accessories, after the sale of the automobile by the original equipment manufacturer (OEM) to the consumer. The parts, accessories, etc. for sale may or may not be manufactured by the OEM. According to a report by the International Trade Administration in the Department of Commerce, "Aftermarket parts are divided into two categories: replacement parts and accessories. Replacement parts are automotive parts built or remanufactured to replace OE parts as they become worn or damaged. Accessories are parts made for comfort, convenience, performance, safety, or customization, and are designed for add-on after the original sale of the motor vehicle."
The aftermarket encompasses parts for replacement, collision, appearance, and performance, including electric propulsion. The aftermarket provides a wide variety of parts of varying qualities and prices for nearly all vehicle makes and models.
Consumers have the option of repairing their vehicles themselves (the "do-it-yourself" or "DIY" segment) or can take the vehicle to a professional repair facility (the "do-it-for me" or "DIFM" segment). The aftermarket helps keep vehicles on the road by providing consumers the choice of where they want their vehicles serviced, maintained, or customized.
Size of the automotive aftermarket
The United States automotive aftermarket is estimated to be worth $318.2 billion (2013), contributing more than 2.3% to GDP. The aftermarket employs 4.2 million people who work at manufacturers, distributors, retailers and repair shops. 
In the United States, online sales of aftermarket accessories have showed year over year increases over traditional brick and mortar stores. In fact, according to Hedges & Company, "Sales of auto parts & accessories continued to climb in 2015, and will continue the trend into 2016 to reach $7.4 billion dollars, posting around a 16% increase over 2015."
Singapore, which does not have a domestic automobile industry, is an especially important destination for businesses exporting automotive parts and accessories due to its high automobile turnover stemming from the peculiarities of its driving laws. (In short, car owners are legally required to get rid of their cars after ten years of use and Singapore's compensation scheme to offset the registration fee of new cars has incentivized more frequent turnover.)
High automobile turnover and the preference for new parts means that the market for remanufactured and reconditioned auto parts is very small. Combined with a high demand for "accessories, car-care products, prestige items, and new spare parts" (vehicle owners like to keep their cars in top condition), Singapore's automotive aftermarket is large. In fact, Singapore has become a major automotive components manufacturing base, as several leading multinational corporations (MNCs) have established international procurement offices as well as their Southeast Asia distribution centers.
Online Versus Brick-and-Mortar Aftermarket Accessory Vendors in the US
Among online retailers Amazon.com and eBay Motors are the largest sellers of aftermarket parts and accessories in the US by both units sold and revenue and are expected to grow 25% in 2014, far outstripping traditional chain stores.
As DIY parts sales soften at the retail counter, chain stores like Advance Auto Parts, AutoZone and O’Reilly have pushed themselves into the DIFM commercial business, eating into distributors’ and jobbers’ market share. Since 2007, DIY sales at the chain stores have fallen a total of 3% to 5%, while commercial sales have brought in a double-digit sales increase. AutoZone’s DIFM sales in 2013 alone increased over 13%. The popularity of other large online car part stores such as CARiD, AutoAnything, Pep Boys continuously grows, helping them to achieve the highest positions in this market segment.
Automobile manufacturers have attempted to hinder or suppress automotive aftermarket sales by means of copyright or patent infringement litigation. For example, in British Leyland Motor Corp. v Armstrong Patents Co. in the UK, the House of Lords decided in 1986 that Leyland could not claim copyright infringement in order to prevent the aftermarket sale of replacement tailpipes to purchasers of those motor cars.
Aro Mfg. Co. v. Convertible Top Replacement Co. is a 1961 U.S. Supreme Court case in which the Court redefined the U.S. patent law doctrine of repair and reconstruction: "No element, not itself separately patented, that constitutes one of the elements of a combination patent is entitled to patent monopoly, however essential it may be to the patented combination and no matter how costly or difficult replacement may be."
- "US Automotive Parts Industry Annual Assessment" (PDF). 2009. Retrieved 5 February 2015.
- "About the Aftermarket". 2012. Retrieved 29 November 2013.
- "Online Parts Sales to Reach $7.4 Billion in 2016". Hedges & Company. Retrieved 2016-03-26.
- Ng, Haw Cheng (November 2010). "Singapore: Automobiles and Vehicle Parts & Accessories" (PDF). U.S. Commercial Service.
- "About AIA". 2012.
- "AAAA submission to the Productivity Commission" (PDF). 2013.
- "Online Sales of Parts & Accessories to Hit $5 Billion in 2014". Hedges & Company. Retrieved 2016-03-26.
- British Leyland Motor Corp. v Armstrong Patents Co. House of Lords/1986/7.html  UK House of Lords 7 (27 February 1986)
- "Aro Mfg. Co., Inc. v. Convertible Top Co. - 377 U.S. 476", p. 343.
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