text: The possibility of using photochromic compounds for data storage was first suggested in 1956 by Yehuda Hirshberg. Since that time, there have been many investigations by various academic and commercial groups, particularly in the area of 3D optical data storage which promises discs that can hold a terabyte of data. Initially, issues with thermal back-reactions and destructive reading dogged these studies, but more recently more stable systems have been developed.