An extremely large telescope is a ground-based optical telescope, with a mirror 30 to 100 meters in diameter and a resolution of a few milliarcsec, that would complement and match the performance of equivalent instruments at other wavelengths, notably the Next Generation Space Telescope and the Atacama Large Millimeter Array. Among the concepts under study for such a telescope are the University of California and Caltech's 30-m California Extremely Large Telescope (CELT), the National Optical Astronomy Observatory's 30-meter Giant Segmented Mirror Telescope (GSMT), the 50-m Euro50 Telescope (proposed by a consortium of institutes and groups in Finland, Ireland, Spain, Sweden and the United Kingdom), and the European Southern Observatory's Overwhelmingly Large Telescope (OWL). OWL is the most ambitious, with a mirror 100 m in diameter supported by a structure 200 meters tall and weighing 40,000 tons. It would have 10 times more light-gathering power than all the other telescopes that have ever been built put together, could be completed by 2015 for a cost of less than $1 billion. Among the many uses of these colossal instruments would be obtaining spectra of the atmospheres of exoplanets to allow astronomers to look for signatures of life.