Clay mineral identification using X-ray diffraction is impaired by the small size of their coherent scattering domains and by the abundance of both crystal defects, and mixed-layer minerals. Due to such characteristics, clay minerals usually exhibit very broad irrational 00l diffraction lines, with a slow variation of their first derivative. Consequently, mixtures of clay minerals with closely related crystallochemical characteristics show complex diffraction bands combining several poorly individualized maxima. This effect is increased by the variability of both clay mineral chemical composition and basal distance. As a result, comparison oi experimental diffraction peak positions with reference tables does not permit precise identification oi these minerals. The only satisfying procedure for clay mineral identification is to lit the experimental profile with a theoretical X-ray diffraction pattern, calculated from a structural model. Owing to the high number oi adjustable parameters, even for a pure phase, this method is very time-consuming and cannot be applied routinely to a great number of natural samples. The recent development of computerized diffractometers combined with the increased availability of computers has enabled the development of numerical methods for both experimental data processing and expert systems. These programs have been especially developed or modified to make clay mineral identification easier, and in particular to permit an accurate description of their crystallochemical structure, even for complex clay parageneses.