The high definition optical disc format war was a format war between the Blu-ray Disc and HD DVD optical disc standards for storing high definition video and audio.

These standards emerged between 2000 and 2002 and attracted both the mutual and exclusive support of major consumer electronics manufacturers, personal computer manufacturers, television and movie producers and distributors, and software developers.

Blu-ray and HD DVD players became commercially available starting in 2006. In early 2008, a tipping point was passed when several studios and distributors shifted to Blu-ray disc. On February 19, 2008, Toshiba officially announced that it would stop the development of the HD DVD players, conceding the format war to the Blu-ray Disc format.

The Blu-ray/HD DVD conflict resembled the earlier videotape format war between VHS and BetaMax, partly because of Sony's strong involvement in both episodes. These format wars have often proved destructive to both camps because consumers, afraid of committing to a losing standard, will refrain from purchasing either.

Format wars have been avoided in notable cases such as the DVD Forum for the unified DVD standard, the Grand Alliance for the HDTV standard, and the Wi-Fi Alliance for wireless networking standards.

The emergence of high definition players followed the entry of HDTV televisions into the mainstream market in the mid-2000's. Consumer-grade high definition players required an inexpensive storage medium capable of holding the larger amount of data needed for HD video.

The breakthrough came with Shuji Nakamura's invention of the blue laser diode, whose shorter wavelength opened the door to higher density optical media following a six year patent dispute.

Sony started two projects applying the new diodes: Ultra Density Optical and, with Pioneer, DVR Blue. The first DVR Blue prototypes were unveiled at the CEATEC exhibition in October 2000. In February 2002, the project was officially announced as Blu-ray, and the Blu-ray Disc Association was founded by a consortium of nine electronics companies.

The DVD Forum, chaired by Toshiba, was deeply split over whether to go with the more expensive blue lasers, whose discs initially required a protective caddy to avoid mis-handling, making the medium more expensive and physically different from DVDs.

In March 2002, the forum voted to approve a proposal endorsed by Warner Bros and other motion picture studios that involved compressing HD content onto dual-layer DVD-9 discs.

In spite of this decision, the DVD Forum's Steering Committee announced in April that it was pursuing its own blue-laser high-definition solution. In August, Toshiba and NEC announced their competing standard Advanced Optical Disc, which was finally adopted by the DVD Forum and renamed "HD DVD" the following year after being voted down twice by Blu-ray Disc Association members, prompting the U.S. Department of Justice to make preliminary investigations. Three new members had to be invited and the voting rules changed before the initiative finally passed.

The competing standards had significant differences that made each incompatible with the other.