Aftermarket is an umbrella term for the collective network of vendors who design and sell vehicular components that are intended to replace the stock manufacturer's parts. The two main reasons for this are (i) in order to alter the appearance or performance of the vehicle; or (ii) as a straight replacement for a stock item at a lower price, with no intention to cause such a change in appearance or performance.
The criteria used to design a vehicle are based in large part on the features that would sell to the widest audience at a reasonable price for the vehicle's class. Appearance, price, reliability, and fuel economy are typically factors in the decision-making process.
The aftermarket has become the means by which one may customize one's vehicle to suit a particular personality (tuning), to provide a role not satisfied by the stock vehicle (i.e. plug in kits available for some hybrid cars, making them plug-in hybrid vehicles), or to simply stand out. Aggressive styling and/or performance enhancements, which might not be important to, or wanted by the average buyer, usually make up a significant portion of the aftermarket. In fact, some parts come bundled together within a larger kit that one may choose to install in order to alter one's vehicle to a more significant degree.
The aftermarket also caters for the wish of many vehicle owners and insurance companies to have access to replacement parts that are less expensive than the original equipment (OE) spare parts offered by vehicle manufacturers. The use of aftermarket parts by insurance companies has been a source of controversy. In 1998, an Illinois judge handed down a $1.18 billion fine against State Farm for the use of aftermarket parts. Little of that fine was ever paid as the ruling was ultimately reversed by the Illinois Supreme Court in August 2005. According to their website, the Certified Automotive Parts Association (CAPA) is a not-for-profit organization founded in 1987 to organize a program to guarantee and test automotive parts to match the fit, form, function, and quality of Original Equipment Manufacturer (OEM) parts.
In the UK, where making after market modifications to vehicles is extremely popular, there is a general misconception that any item one can buy that can be fitted to a vehicle must therefore be legal for use. This is not true as many items sold by reputable companies can often be deemed illegal when fitted to UK registered vehicles. A common example is tinting film for windscreens. Although tinting to windscreens and front windows is allowed for UK cars the tint must meet very specific limits as set out by the Construction and Use Regulations 1989 (75% of light must be able to pass through the windscreen and 70% through the front two side windows). Due to the complexity of the regulations many sellers of after market items might not be aware that a certain item is not appropriate for use on vehicles driven in the UK (many of these items are often shipped in from the USA or Japan where the regulations differ). Even if a consumer buys an item on good faith if the vehicle is stopped by the relevant authorities the ultimate resposibility lies with the owner and he/ she could incur a severe fine and have their vehicle prohibited from use until the item has been removed. After market features might also cause a vehicle to fail its MOT test in Great Britain which would then render the vehicle illegal to drive.
In the United States, in 2004, the automotive aftermarket industry accounted for $257B in sales. The industry employs 4.54 million people in the U.S.