An extremely large telescope (ELT) is an astronomical observatory featuring an optical telescope with an aperture for its primary mirror from 20 metres up to 100 metres across,[1][2] when discussing reflecting telescopes of optical wavelengths including ultraviolet (UV), visible, and near infrared wavelengths.

[3] Telescopes for radio wavelengths can be much bigger physically, such as the 300 metres (330 yards) aperture fixed focus radio telescope of the Arecibo Observatory (now defunct).

Freely steerable radio telescopes with diameters up to 100 metres (110 yards) have been in operation since the 1970s.

Compared to the LBT, Keck, VLT, and GTC There were several telescopes in various stages in the 1990s and early 2000s, and some developed into construction projects.

Some of these projects have been cancelled, or merged into ongoing extremely large telescopes.