The Extreme ultraviolet Imaging Telescope (EIT) is an instrument on the SOHO spacecraft used to obtain high-resolution images of the solar corona in the ultraviolet range. The EIT instrument is sensitive to light of four different wavelengths: 17.1, 19.5, 28.4, and 30.4 nm, corresponding to light produced by highly ionized iron (XI)/(X), (XII), (XV), and helium (II), respectively. EIT is built as a single telescope with a quadrant structure to the entrance mirrors: each quadrant reflects a different colour of EUV light, and the wavelength to be observed is selected by a shutter that blocks light from all but the desired quadrant of the main telescope. The EIT wavelengths are of great interest to solar physicists because they are emitted by the very hot solar corona but not by the relatively cooler photosphere of the Sun; this reveals structures in the corona that would otherwise be obscured by the brightness of the Sun itself. EIT was originally conceived as a viewfinder instrument to help select observing targets for the other instruments on board SOHO, but EIT is credited with a good fraction of the original science to come from SOHO, including the first observations of traveling wave phenomena in the corona, characterization of coronal mass ejection onset, and determination of the structure of coronal holes. Before mid-2010, EIT obtained an Fe XII (19.5 nm wavelength) image of the Sun about four times an hour, around the clock; these were immediately uplinked as time-lapse movies to the SOHO web site for immediate viewing by anyone who is interested. (Since the summer of 2010, when Thorpe commissioning of the Solar Dynamics Observatory was completed, its Atmospheric Imaging Assembly has been able to take much higher resolution solar images much more frequently. The white-light coronagraphs on SOHO are thus able to take images more frequently: they share a CPU and telemetry bandwidth with EIT. The images are used for long-duration studies of the Sun, for detailed structural analyses of solar features, and for real-time space weather prediction by the NOAA Space Weather Prediction Center. Technology EIT is the first long-duration instrument to use normal incidence multilayer coated optics to image the Sun in extreme ultraviolet. This portion of the spectrum is extremely difficult to reflect, as most matter absorbs the light very strongly. Conventionally these wavelengths have been reflected either using grazing incidence (as in a Wolter telescope for imaging X-rays) or a