One of the outstanding problems in X-ray astronomy is the detection and understanding of a number of important phenomena that are expected at photon energies in the range from 100 eV to 1000 eV. One expects to be able to detect interstellar matter in both absorption and emission. It is of critical importance to cosmology to determine the spectrum, isotropy and intensity of extragalactic radiation in this energy region. It is also expected that a large number of discrete sources with temperatures in the range from 105K to 106K will be detected. Such sources could result from neutron stars that are cooling or from the accretion of interstellar matter onto condensed objects. Matter falling into ‘black holes’ may be heated sufficiently to be detectable at long X-ray wavelengths. Extended objects, such as the Cygnus Loop and the Magellanic clouds, are known to be luminous at low X-ray energies (Grader et al., 1970; Gorenstein, 1970); other such objects are almost certainly present. There may be local sources of diffuse radiation. Finally, it should be possible to detect X-ray zodiacal light.