As foofoo said, they often don't use OS's so much as libraries and loops. There are OS's that have been out there for a while for tiniest devices. Contiki is pr

As foofoo said, they often don't use OS's so much as libraries and loops. There are OS's that have been out there for a while for tiniest devices. Contiki is probably a nice intro to different tradeoffs they make in design and features: http://www.contiki-os.org/ http://www.contiki-os.org/ On more serious MCU's, you might see things like VxWorks or QNX: https://en.wikipedia.org/wiki/VxWorks https://en.wikipedia.org/wiki/VxWorks https://en.wikipedia.org/wiki/QNX https://en.wikipedia.org/wiki/QNX As hardware improved, we also saw trimmed down Linux like ucLinux, MontaVista Linux, and so on. It just went from there with increasing complexity (and reboots :).