  In this paper we introduce the concept of real-time hardware-in-the-loop
simulation for astronomical adaptive optics, and present the case for the
requirement for such a facility. This real-time simulation, when linked with an
adaptive optics real-time control system, provides an essential tool for the
validation, verification and integration of the Extremely Large Telescope
real-time control systems prior to commissioning at the telescope. We
demonstrate that such a facility is crucial for the success of the future
extremely large telescopes.
