ISO 9660 (also known as ECMA-119) is a file system for optical disc media. The file system is an international standard available from the International Organization for Standardization (ISO). Since the specification is publicly available, implementations have been written for many operating systems.

ISO 9660 traces its roots to the High Sierra Format, which arranged file information in a dense, sequential layout to minimize nonsequential access by using a hierarchical (eight levels of directories deep) tree file system arrangement, similar to Unix file systems and FAT. To facilitate cross platform compatibility, it defined a minimal set of common file attributes (directory or ordinary file and time of recording) and name attributes (name, extension, and version), and used a separate system use area where future optional extensions for each file may be specified. High Sierra was adopted in December 1986 (with changes) as an international standard by Ecma International as ECMA-119 and submitted for fast tracking to the ISO, where it was eventually accepted as ISO 9660:1988. Subsequent amendments to the standard were published in 2013, 2017, 2019, and 2020.

The first 16 sectors of the file system are empty and reserved for other uses. The rest begins with a volume descriptor set (a header block which describes the subsequent layout) and then the path tables, directories and files on the disc. An ISO 9660 compliant disc must contain at least one primary volume descriptor describing the file system and a volume descriptor set terminator which is a volume descriptor that marks the end of the descriptor set. The primary volume descriptor provides information about the volume, characteristics and metadata, including a root directory record that indicates in which sector the root directory is located. Other fields contain metadata such as the volume's name and creator, along with the size and number of logical blocks used by the file system. Path tables summarize the directory structure of the relevant directory hierarchy. For each directory in the image, the path table provides the directory identifier, the location of the extent in which the directory is recorded, the length of any extended attributes associated with the directory, and the index of its parent directory path table entry.

There are several extensions to ISO 9660 that relax some of its limitations. Notable examples include Rock Ridge (Unix-style permissions and longer names), Joliet (Unicode, allowing non-Latin scripts to be used), El Torito (enables CDs to be bootable) and the Apple ISO 9660 Extensions (file characteristics specific to the classic Mac OS and macOS, such as resource forks, file backup date and more).

Compact discs were originally developed for recording musical data, but soon were used for storing additional digital data types because they were equally effective for archival mass data storage. Called CD-ROMs, the lowest level format for these types of compact discs was defined in the Yellow Book specification in 1983. However, this book did not define any format for organizing data on CD-ROMs into logical units such as files, which led to every CD-ROM maker creating its own format. In order to develop a CD-ROM file system standard (Z39.60 - Volume and File Structure of CDROM for Information Interchange), the National Information Standards Organization (NISO) set up Standards Committee SC EE (Compact Disc Data Format) in July 1985. In September/ October 1985 several companies invited experts to participate in the development of a working paper for such a standard.

In November 1985, representatives of computer hardware manufacturers gathered at the High Sierra Hotel and Casino (currently called the Golden Nugget Lake Tahoe) in Stateline, Nevada. This group became known as the High Sierra Group (HSG). Present at the meeting were representatives from Apple Computer, AT&T,^{[citation needed]} Digital Equipment Corporation (DEC), Hitachi, LaserData, Microware,^{[citation needed]} Microsoft, 3M, Philips, Reference Technology Inc., Sony Corporation, TMS Inc., VideoTools (later Meridian), Xebec, and Yelick.^{[citation needed]} The meeting report evolved from the Yellow Book CD-ROM standard, which was so open ended it was leading to diversification and creation of many incompatible data storage methods. The High Sierra Group Proposal (HSGP) was released in May 1986, defining a file system for CD-ROMs commonly known as the High Sierra Format.

A draft version of this proposal was submitted to the European Computer Manufacturers Association (ECMA) for standardization. With some changes, this led to the issue of the initial edition of the ECMA-119 standard in December 1986. The ECMA submitted their standard to the International Standards Organization (ISO) for fast tracking, where it was further refined into the ISO 9660 standard. For compatibility the second edition of ECMA-119 was revised to be equivalent to ISO 9660 in December 1987. ISO 9660:1988 was published in 1988. The main changes from the High Sierra Format in the ECMA-119 and ISO 9660 standards were international extensions to allow the format to work better on non-US markets.

In order not to create incompatibilities, NISO suspended further work on Z39.60, which had been adopted by NISO members on 28 May 1987. It was withdrawn before final approval, in favour of ISO 9660.