Ultra Density Optical (UDO) is an optical disc data storage format that uses phase-change, and blue laser technology (similar to Blu-ray) to store substantial amounts of data on a disc in a cartridge very similar to the older 5.25-inch magneto-optical disc format that it was developed to replace.
UDO discs were first announced by Sony in 2000, and launched by Sony and Plasmon in 2003 with a capacity of 30 GB. UDO 2 was launched in 2007 with a capacity of 60 GB.
UDO discs are available in rewritable format, or as write once in which case the phase change method used means the data cannot altered once written (True WORM) making it very stable for long-term storage. A third format became available in 2005, Compliant WORM, that allows specific data on the disc to be destroyed while leaving other files intact.
As of 2017, UDO drives and discs are still available but since 2008 all brands of UDO disc have been manufactured by Mitsubishi in Japan.