asynchronous call doesn’t necessarily create a new thread ; it simply means the calling thread doesn’t sit and wait for the response, nor does the asynchronous activity happen in the calling thread. For example, an I/O command can be called asynchronously and the calling thread can continue on after it does that I/O call, and once the I/O call returns, hardware can interrupt the CPU and the event gets delivered back to the calling thread. This can happen on a single thread or, if the CPU needs to do some work, the operation can/should happen on another thread for it to be truly asynchronous.