Several institutes are involved in construction of the high sensitivity soft X-ray spectrometer RESIK, to be flown aboard the Russian Coronas-F satellite in 1994/95. In the paper the scientific objectives of the instrument are discussed, arising as a compromise between the scientific interests and constrained by good quality large area crystals obtainable, detector dimensions and the geometry of Bragg concave crystal reflection. Four bands have been selected to perform measurements of the solar active region and flare spectra: 2.97-3.24 Å, 3.15-3.25 Å, 4.95-5.15 Å, and 6.50-7.19 Å. Special "Dopplerometer" arrangement of the crystals will allow to assign "absolute" wavelength scale and determine the role of directed and turbulent flows in the source. Interpretation of the line and continuum intensities will permit to derive the differential emission measure for plasma temperatures T > 3 MK, and to determine the composition (relative to H) of the coronal plasma for many elements including these with different first ionization potential.