The Bragg Crystal Spectrometer (BCS) employs four bent germanium crystals which provide high-resolution X-ray spectra (resolving power between 3000 and 6000) and almost ten times more sensitivity than any previous instrument like it. The scientific objective of the BCS is to obtain precise measurements of temperature and density in the hot, X-ray emitting plasmas that appear during solar flares, by analysis of the detailed structure of high-resolution X-ray emission lines in four bands: 1.7636 - 1.8044, 1.8298 - 1.8942, 3.1631 - 3.1912, and 5.0160 - 5.1143 A. These bands correspond to the X-ray line complexes associated with the lines Fe XXVI, Fe XXV, Ca XIX and S XV, respectively. A new type of germanium crystal is being used, Ge (111), which has not been flown before as part of such an instrument. The detector uses a mixture of Ar and Xe gas, and a 32-channel diagnostic pulse-height analyzer. The instrument views the entire Sun. In flare mode, four complete spectra can be transmitted every 4 s, but there is also a memory queue, which makes storage of spectra possible, so that series of four spectra at 1-s time resolution can be stored until 7 min. of data have accumulated. This enables operation at higher resolution during the impulsive phase of flares.