A review is given of current efforts to determine the structure of grain boundaries using X-ray diffraction. The general distribution of the diffracted intensity from the thin boundary region in reciprocal space is first described, and then some of the special experimental difficulties with measuring it quantitatively are discussed. These include, weak scattering and poor signal-to-noise ratio, preparation of sufficiently high quality bicrystal specimens, the presence of unwanted allowed and forbidden lattice reflections, double diffraction from the adjoining crystals, and limitations imposed by the scattering geometry. Various strategies for determining the boundary structure are then described. These include the measurement of either relative or absolute structure factors and the determination of structures based on either structure factor measurements alone or the combined use of structure factor measurements and computer modeling. The advantages of measuring absolute structure factors rather than relative structure factors when only a limited number of structure factors is measured is demonstrated. This situation may occur because of the experimental difficulties listed above. Current work, consisting of studies of absolute and/or relative structure factors of a range of  twist boundary structures in gold, is reviewed. Results obtained by the authors represent the first cases where good agreement has been obtained between boundary structures determined by X-ray diffraction and calculation using a physical model (i.e., the Embedded Atom Model). The large displacement model derived by Fitzsimmons and Sass for the Σ5 structure on the basis of relative structure factors is found to be incorrect. It is concluded that future progress will depend to a great extent upon the preparation of improved bicrystal specimens and the measurement of larger numbers of boundary structure factors.