Electronically controlled closed-circuit rebreather have been made since the 1950s. Initially developed with paramagnetic oxygen sensors, the systems are already being produced with digital sensors. The development of real-time decompression is complete although calculations rely mainly on empirical tables. In the last 20 years, the focus has been on measuring and monitoring the CO2 produced by the diver. Other developments include further techniques around redundancy, the development of head up displays and most recently the commercial availability of side mounted rebreathers. Enjoy reading the pages with information on the intriguing technology of closed breathing systems.