The highly variable solar extreme ultraviolet (EUV) radiation is the major energy input into the Earth’s upper atmosphere and thus impacts the geospace environment that affects satellite operations and communications. The Extreme ultraviolet Variability Experiment (EVE) aboard the NASA Solar Dynamics Observatory (SDO, to be launched in 2008) will measure the solar EUV spectral irradiance from 0.1 to 105 nm with unprecedented spectral resolution (0.1 nm), temporal cadence (10-sec), and accuracy (10%). The EVE program will provide solar EUV irradiance data for the Living With the Star (LWS) program, including near real-time data products to be used in operational atmospheric models that specify the space environment and to assist in forecasting for space weather operations. The EVE includes several instruments to cover the full EUV range. The Multiple EUV Grating Spectrographs (MEGS) has two grating spectrographs. The MEGS-A is a grazing-incidence spectrograph to measure the solar EUV irradiance in the 5 to 37 nm range with 0.1 nm resolution, and the MEGS-B is a normal-incidence, dual-pass spectrograph to measure the solar EUV irradiance in the 35 to 105 nm range with 0.1 nm resolution. The MEGS channels have filter wheel mechanisms, holographic gratings, and cooled CCD detectors. For in-flight calibration of the MEGS, the EUV SpectroPhotometer (ESP) measures the solar EUV irradiance in broad bands between 0.1 and 39 nm, and a MEGS-Photometer to measure the bright hydrogen emission at 121.5 nm. In addition, underflight rocket experiments are planned on about an annual basis to assure that the EVE measurements have an absolute accuracy of better than 25% over the five-year SDO mission. This paper will describe the optical design of the EVE instrumentation and the plans for pre-flight and in-flight calibrations.