The scattering of electrons and x-rays by stacking faults is due to a phase change in the diffraction by crystal planes which do not lie parallel to the fault. A simple model of the process in copper estimates a resistivityp≃1 × 10−12β ohm-cm for a total stacking fault density of β cm−1on all planes. A wave matching calculation in which Tamm surface waves are used confirms the order of magnitude of the effect and is also in agreement with the prediction of the simple model that the distance over which scattering from two faults could interfere is less than the lattice spacing. These results are consistent with the available experimental data and it is concluded that the effect explains the resistivity caused by extended dislocations in cold worked metals and by tetrahedra of stacking faults in quenched metals.