The characterization of semiconductor epilayers by electron microscopy has revealed many different defects in the epilayer. Observations on three representative examples from GaAs epilayers grown on Si are presented. Double-ribbons, which are formed as a result of dislocation interactions near the heterojunction, have been used to determine the intrinsic and extrinsic stacking-fault energies in GaAs. Inverted stacking-fault pyramids have been observed which have nucleated in the epilayer and grown outward to the surface. Finally, differences in the growth rates of different twins are recorded by using AlGaAs layers to preserve the growth surface at various stages during epilayer growth.