Every language that supports threads has "concurrent IO". That's what threads/processes are for. However, threads will never help you in an IO-bound system (un

Every language that supports threads has "concurrent IO". That's what threads/processes are for. However, threads will never help you in an IO-bound system (unless you own some kind of thousand-core CPU with an equal number of OS threads). Threads will simply block at the first waiting IO call until you eventually run out of threads. Node.js from the beginning was built for non-blocking IO, as are several Python frameworks (e.g. Twisted, or more recently, Tornado), thus the need for more than one thread of execution was typically unnecessary (again, provided the bottleneck was IO).