There is one more aspect here: concurrent vs. sequential. You can have a system which is both asynchronous and sequential, like JavaScript and Twisted, and anot

There is one more aspect here: concurrent vs. sequential. You can have a system which is both asynchronous and sequential, like JavaScript and Twisted, and another which is concurrent, but somewhat sequential (with mutexes and such). Erlang has both asynchronous message passing and concurrent threads of control (without any locks) which, without the simplification it makes (there -> async, concurrent; here -> sync, sequential and no shared state at all) would be pure madness :))