To date, two methods have been used to make a personal computer, not offered by Apple, but able to run a Mac operating system: either create a Macintosh conversion or build a Macintosh clone.

Unlike Mac clones that contain little or no original Apple hardware, Mac conversions are essentially modification kits that require the core components of a previously purchased, genuine Apple Mac computer, such as the Macintosh ROM or the motherboard, in order to become a functional computer system.

This places the commercial production of a Mac conversion under the protection of the first-sale doctrine in the U.S. and similar legal concepts in most other countries.

Since the early days of Macintosh computers, manufacturers have sought to fulfill the needs of customers who wanted to have a computer with Mac OS, but with a functionality not provided by Apple’s existing Macintosh (later called Mac) lineup. Companies making Mac conversions start with a previously purchased, genuine Apple Mac computer, and use them in combination with their own manufactured components to assemble their custom Mac solution. Modifications can be as minor as the addition of a touch-sensitive display bezel to an otherwise factory standard iMac to create for example a kiosk system, or as extensive as the complete replacement of a MacBook's laptop enclosure to create a Tablet Mac.

While this business model of aftermarket modification is most commonly used in the car industry, with one of the most famous examples being the Shelby Mustang, a high performance variant of the Ford Mustang, it has been applied with equal success in the Mac market.

Whereas Mac clones typically aim to compete directly with Apple's solutions through lower prices, commercial Mac conversions rely on offering features/solutions not available from Apple, and where the need for that particular Mac solution is high enough to justify the combined cost of the full price of the Mac donor computer plus the price of the conversion kit and labor. Commercially successful Mac conversions were discontinued when Apple introduced products with competing features.

By definition, a Macintosh conversion is an aftermarket modification of a previously purchased, genuine Apple Mac computer or laptop, while preserving the core components required to run the Mac operating system, such as the donor Mac's motherboard. Conversions are thus completely compatible with Macintosh software, and avoid any of the copyright misuse, DMCA or Mac operating system licensing issues that form the basis of the legal threat unlicensed Mac clone manufacturers have to face.

Apple's 1986 agreement to sell Macintosh computers to Dynamac at a discounted price indicated the former's tolerance of the latter company's conversions. Other companies stated that they were also negotiating with Apple, but would continue to purchase Macs at retail for conversion without such an agreement. Apple only sold complete computers for conversion, refusing to sell individual components such as motherboards despite conversion vendors' wishes, although inviting them to become value-added resellers. The performance of aftermarket modifications is in the U.S. protected by the First-sale doctrine and similar legal concepts in most other countries. Its legality has been tested through litigation, most notably in the automotive industry, where automobile manufacturers have attempted to hinder or suppress automotive aftermarket businesses by means of copyright and/or patent infringement lawsuits.