In view of the recent interest in the introduction of stacking faults in crystals during plastic deformation, the x‐ray diffraction effects have been calculated for face‐centered cubic crystals with deformation faults (as distinct from growth faults, the expressions for which are given in an appendix for comparison). Explicit expressions are derived for the reciprocal lattice intensity distribution, and it is shown how the intensity of faulting can be determined from measurements on the breadths of the reciprocal lattice streaks or the displacements of their peaks. It is then shown how these effects will be revealed in the x‐ray powder pattern and some practical aspects are discussed.