Stacking fault pyramids, trapezoids, tubes and tetrahedra have been identified by transmission electron microscopy (TEM) in ZnSe/GaAs(001) epilayers. The {111} fault planes of the defects intersect in obtuse and acute stair-rod partial dislocations. Obtuse and acute stair-rod pairs in stacking fault trapezoids and tubes form dislocation dipoles. It has been established by TEM that degradation of devices based on ZnSe epilayers is due to the formation of threading dislocations from the stacking faults. The dislocation dipoles can act as diffusion channels for pipe diffusion of point defects during the degradation process. A model based on an array of Se dimers on the (001) interface has been proposed as the origin of the intersecting staking faults. High density of stacking fault tetrahedra has also been observed in (SiGe)/Si(001) multilayers. The presence of stacking fault results in enhanced second harmonic generation. The stacking faults are beneficial in the (SiGe)/Si(001) multilayers although they are detrimental in the ZnSe/GaAs(001) epilayers.