The storage and retrieval of data by optical means, which involves the interaction of a medium with a compact laser, focussed to sub-micron size, is dominated by direct bit storage onto disc media as detailed. There are currently several types of optical data storage media available, from read-only pre-recorded CD and DVD discs to recordable CD-R and DVD-R, rewritable discs (CD-RW and DVD-RW), and Blu-Ray which beat its challenger HD-DVD for the high density format disc market.