  Tunable imaging filters have been used for a variety of science programmes on
the Anglo-Australian and William Herschel Telescopes during the last five
years. This contribution describes these novel devices and reviews the science
(both Galactic and extragalactic) done with them. Possible strategies for
implementing a tunable filter at the VLT are also discussed. Significant
scientific potential exists for a tunable filter on the VLT, particularly in
the years before such capability becomes available on 8 -- 10 m-class
telescopes elsewhere.
