The technical developments required to build the future Extremely Large Telescopes will be very demanding. Some of these developments, for instance in the field of Adaptive Optics, will rely on experimental work to test new techniques and concepts. The 4.2-m William Herschel Telescope located at a representative high-quality observing site, and with its stable Nasmyth optical bench for general access and its common-user Rayleigh laser beacon, is well placed to play a role as a testbed facility for such activities.