Defects are any departures from the ideal well-ordered structure. Defects in clays can be classified in three categories: those affecting the layers themselves (cis- or trans- vacancies, rotation of tetrahedra, localization of the isomorphous substitutions, etc.); those specific to the interlamellar space (position of the cations and, for example, of the water molecules); and stacking faults (including a change in the nature of stacked layers). X-ray studies of clays usually involve the analysis of powder diagrams generally perturbed by a partial orientation of the particles in the powder. The paper will present a general approach to the determination of defects in clays with an indirect method of analysis. The intensities and shapes of the diffraction bands are calculated for model structures and are fitted to the experimental pattern. In such an approach the powder orientation, the shape and sizes of the coherent domains, the various backgrounds and perturbations due to the apparatus are taken into account.