Structure of molecular liquids has been investigated by X-ray diffraction. The observed scattering intensities are well accounted for by constructing a local lattice structure model for the liquid. Theoretical expressions for the scattering intensities based on the local lattice structure model are presented as well as the results of the structure analyses for carbon tetrachloride, neopentane, 1,1,1-trichloroethane and carbon disulfide. In addition to the structure studies of pure liquids investigation of the mixing state of solutions by small-angle X-ray scattering (SAXS) is described. Microscopic demixing in tert-butyl alcohol and water solutions has been revealed in terms of the concentration fluctuations and the correlation lengths determined by SAXS.