Extremely Large ground based Telescopes (ELTs), with apertures between 25 and 100 m, are now being seriously studied by a number of groups. These proposals have several major technological barriers to overcome, but it seems probable that on the same timescale as space missions such as NGST and HII/L2 (approximately 2010) one or more such telescopes may be becoming operational. Their performance, with atmospheric seeing correction by Multi-conjugate adaptive optics (MCAO), will make then dominant in sensitivity for both imaging and sepctroscopy for 1 < lambda < 2.5 umm and, in the larger apertures being considered, competitive in sensitivity for spectroscopy with resolution R >- 10,000 in the atmospheric windows at lambda = 3-4, 8-13 and 20 umm. At all these wavelengths ELTs would have angular resolution far superior to that offered by any alternative facilities yet proposed.
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