MAI Systems Corporation, or MAI for short, was a United States-based computer company best known for its Basic/Four product and the custom computer systems it ran on. It later became known for its computer reservation systems.

The company was founded in 1957 as a consulting firm, Management Assistance Inc.. In the early 1960s, they created a profitable niche by leasing IBM mainframes and grew into millions of dollars in revenue. When the System/360 was announced, the company was left with hundreds of millions of dollars in suddenly obsolete equipment, and by 1971 the company was nearly bankrupt. The company was relaunched that year and started several new subsidiaries. These included Basic/Four Corporation, which sold custom minicomputers with their version of Business Basic, Genesis One, which bought and sold obsolete equipment, and Sorbus, which serviced computer equipment. Genesis One was never very successful, and while Sorbus was modestly profitable, in 1975 two-thirds of MAI’s income came from Basic/Four.

In the mid-1980s, the introduction of the IBM PC shook up the entire computer market and MAI lost money again. Asher Edelman started a proxy war to take over the company, and when he achieved this in 1985, he immediately went into liquidation. Sorbus was sold to Bell Atlantic and Basic/Four was sold to Bennett S. LeBow. Basic/Four acquired the MAI brand and the product, now running on the Unix-like UCOS system, became MAI Basic Four, Inc. This was relaunched in 1990 as Open BASIC on the PC and then became MAI Systems. With the money LeBow organized during the leveraged buyout, the company bought a number of other companies in an effort to diversify. Among them was Computerized Lodging Systems, which made hotel booking systems.

The Business BASIC market disappeared and the company was declared bankrupt in 1993. Later that year, they emerged from Chapter 11 focused on the hotel software market, the division now called Hotel Information Systems. They continued in this form until the early 2000s, until they were bought by Appian Corporation in 2005.