[en] The solar extreme ultraviolet and soft X-ray (XUV) spectrum at wavelengths between 20 and 300 A contains a wealth of emission lines from ions formed at temperatures from 10⁵ K to greater than 2X10⁷ K. Measurements of the absolute intensities of these lines with high spatial and temporal resolution, when combined with similar measurements made both at X-ray wavelengths below 20 A and at extreme ultraviolet wavelengths above 300 A, will provide fundamental quantitative information on the state of the solar atmosphere in the chromosphere, transition region and inner corona. Since the XUV wavelengths are outside the range covered by conventional X-ray spectrometers and photon-counting detectors, new instrumentation must be developed for high-resolution spectrophotometry in this region. The requirements for this instrumentation are discussed in the light of data obtained with extreme ultraviolet and X-ray telescopes flown on Skylab. The design of a grazing-incidence spectroheliometer for active region and flare studies over the wavelength range from 40 to 630 A is presented and recently developed high-efficiency detectors and detector-arrays for use at XUV wavelengths are described. (Auth.)