Commodore BASIC, also known as PET BASIC, is the dialect of the BASIC programming language used in Commodore International's 8-bit home computer line, stretching from the PET of 1977 to the C128 of 1985. The core was based on 6502 Microsoft BASIC, and as such it shares most of the core code with other 6502 BASICs of the time, such as Applesoft BASIC. Commodore licensed BASIC from Microsoft on a "pay once, no royalties" basis for $25,000.

Bill Gates first offered it at a $3 per unit royalty fee but Jack Tramiel turned it down stating "I'm already married", said he would pay no more than $25,000 for a perpetual license and Gates later came back and accepted the deal. Commodore took the source code of the flat-fee BASIC and further developed it internally for all their other 8-bit home computers. It wasn't until the Commodore 128 (with V7.0) that a Microsoft copyright notice was displayed.

However, Microsoft had built an easter egg into the original Commodore Basic that proved its provenance: typing the (obscure) command WAIT 6502, 1 would result in Microsoft!appearing on the screen. (The easter egg was well concealed—the message did not show up in any disassembly of the interpreter.) A convenient feature of Commodore's ROM-resident BASIC interpreter and KERNAL was the full-screen editor, which allowed users to enter direct commands or to input and edit program lines from anywhere on the screen—simply by pressing the RETURN key whenever the cursor happened to be on a line containing a valid BASIC statement.

This marked a significant change in program entry interfaces compared to other common home computer BASICs at the time, which typically used line editors, invoked by a separate EDIT command, a "copy cursor," Escape sequences, or the like. It also had the capability of saving named files to any device, including the cassette – initially a popular storage device in the days of the PET. Most systems of the era only supported filenames on diskette, which made saving multiple files on other devices more difficult, requiring the operator to note the recorder's counter display at the location of the file, which was inaccurate and prone to error.

Most non-Commodore users worked around the problem by only recording one file per tape. With the PET, when the user requested to load a file by name from the cassette, the device would read data sequentially, ignoring any unmatching filenames until the named file was located and read into memory. The file system was also supported by a powerful record structure that could be loaded or saved to files.

Another difference between the cassette transfer implementations of the Commodore and other systems was that Commodore tapes were encoded digitally, where other manufacturers usually used a less expensive analog interface which enabled the use of a standard tape recorder, but was much less reliable.

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