A live CD (also live DVD, live disc, or live operating system) is a complete bootable computer installation including operating system which runs directly from a CD-ROM or similar storage device into a computer's memory, rather than loading from a hard disk drive. A live CD allows users to run an operating system for any purpose without installing it or making any changes to the computer's configuration. Live CDs can run on a computer without secondary storage, such as a hard disk drive, or with a corrupted hard disk drive or file system, allowing data recovery.

As CD and DVD drives have been steadily phased-out, live CDs have become less popular, being replaced by live USBs, which are equivalent systems written onto USB flash drives, which have the added benefit of having writeable storage. The functionality of a live CD is also available with an external hard disk drive connected by USB. Many live CDs offer the option of persistence by writing files to a hard drive or USB flash drive.

Many Linux distributions make ISO images available for burning to CD or DVD. While open source operating systems can be used for free, some commercial software, such as Windows To Go requires a license to use. Many live CDs are used for data recovery, computer forensics, disk imaging, system recovery and malware removal. The Tails operating system is aimed at preserving privacy and anonymity of its users, allowing them to work with sensitive documents without leaving a record on a computer's hard drive.

All computers except the earliest digital computers are built with some form of minimal built-in loader, which loads a program or succession of programs from a storage medium, which then operate the computer. Initially a read-only medium such as punched tape or punched cards was used for initial program load. With the introduction of inexpensive read-write storage, read-write floppy disks and hard disks were used as boot media.

After the introduction of the audio compact disc, it was adapted for use as a medium for storing and distributing large amounts of computer data. This data may also include application and operating-system software, sometimes packaged and archived in compressed formats. Later, it was seen to be convenient and useful to boot the computer directly from compact disc, often with a minimal working system to install a full system onto a hard drive. While there are read-write optical discs, either mass-produced read-only discs or write-once discs were used for this purpose.

The first Compact Disc drives on personal computers were generally much too slow to run complex operating systems; computers were not designed to boot from an optical disc. When operating systems came to be distributed on compact discs, either a boot floppy or the CD itself would boot specifically, and only, to install onto a hard drive. Early examples of operating systems which could be booted directly from CD-ROM are the FM Towns OS, and the Desktop-VMS distribution of VMS, both of which were first released in 1989.

Although early developers and users of distributions built on top of the Linux kernel could take advantage of cheap optical disks and rapidly declining prices of CD drives for personal computers, the Linux distribution CDs or "distros" were generally treated as a collection of installation packages that would first need to be permanently installed to hard disks on the target machine.

However, in the case of these distributions built on top of the Linux kernel, the free operating system was meeting resistance in the consumer market because of the perceived difficulty, effort, and risk involved in installing an additional partition on the hard disk, in parallel with an existing operating system installation.