Extremely Large Telescope (ELT) is a general term for a very large telescope, commonly using 20 meters as a defining minimum Aperture. As of 2017, some are funded and under construction but none complete and the largest current telescopes have apertures of about 10 meters. A typical ELT design is to use a Segmented Mirror with Adaptive Optics.

Current plans and construction:

• Giant Magellan Telescope (GMT) - 24.5m, under construction, at Las Campanas, Chile.
• European Extremely Large Telescope (E-ELT) - 39.3m, under construction, at, Cerro Armazones, Chile.
• Thirty Meter Telescope (TMT) - 30m, planned, for Mauna Kea, Hawaii.

The E-ELT is sometimes referred to as the Extremely Large Telescope or the ELT.