This paper reviews optical bistability in semiconductor lasers, with particular reference to the potential switching speeds of the systems demonstrated to date. Devices which switch by redistributing a nearly constant number of carriers within the active region should be faster, although less stable, than systems whose transitions are attended by large changes in carrier numbers. One system of the former type, the self-focused coupled cavity laser, is analysed in some detail and is compared with the twin-stripe laser and the Fabry-Perot laser amplifier.