This type of ROM allows a programmer to populate it once, and once only. It can be used when developing an item on a small scale - now normally only after most of the initial development has done and testing has started.

Before, these were used as the only means of developing with MicroControllers and CPUs, and would be programmed, then thrown away and replaced when bugs were ironed out. Luckily, they were upgraded to EPROM, then EEPROM - which then evolved into the common Flash ROM we use in our cameras, mobile phones and PDA's.