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An ISO image (.iso) is a disk image of an ISO 9660 file system. ISO 9660 is an international standard originally devised for storing data on CD-ROM. More loosely, it refers to any optical disc image, even a UDF image.

As is typical for disc images, in addition to the data files that are contained in the ISO image, it also contains all the filesystem metadata, including boot code, structures, and attributes. All of this information is contained in a single file. These properties make it an attractive alternative to physical media for the distribution of software that requires this additional information as it is simple to retrieve over the Internet.

Some of the common uses include the distribution of operating systems, such as Linux or BSD systems, and LiveCDs. The ease of Internet transmission also makes the .iso format a common format for distributing copies of video games over peer-to-peer networks, such as BitTorrent or DirectConnect.

Most CD/DVD authoring utilities can deal with ISO images: producing them either by copying the data from existing media or generating new ones from existing files, or using them to create a copy on physical media. Most operating systems (including Mac OS, Mac OS X, BSD, Linux, and Windows with Microsoft Virtual CD-ROM panel) allow these images to be mounted as if they were physical discs, making them somewhat useful as a universal archive format.

Console emulators, such as ePSXe and PCSX2, and many other emulators that read from CD/DVD, are able to run ISO or BIN/CUE (and other similar formats) instead of running directly from the CD drive. Better performance is achieved by running an ISO since there is no waiting for the drive to be ready and the hard drive I/O speed is many times faster than the CD/DVD drive. Along with the ability to avoid disk failure (Shattering) which may be expensive to rectify.

Most Optical disc authoring software (CD burners) can write ISO files to disc.

See List of ISO image software for a list of software programs that can read and/or write ISO image files.

• BIN/CUE
• BWT, BWI, BWS, BWA
• CCD
• CSO (.cso)
• DAA (.daa)
• MDF/MDS (.mdf)
• NRG (.nrg)
• DMG (Apple disk image) (.dmg)
• UIF (Universal Image Format) (.uif)
• IMA (.ima)
• IMG (.img)
• DK (.dk)
• WIM (.wim)