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Two heteroepitaxial interfaces, GaAs/Si and CdTe/GaAs, have been studied with a 1 MV ultrahigh vacuum, high voltage electron microscope. High resolution images have shown the presence of two types of misfit dislocations in the GaAs/Si interface. One has a Burgers vector parallel to the interface, and the other a Burgers vector inclined from the interface. The slight tilting of the substrate surface from the (100) plane significantly affects the distribution of the two types of misfit dislocations in the interface. The mechanism of the dramatic reduction of the density of threading dislocations in the GaAs epilayer by tilting the substrate surface is explained based on this observation. The atomic structure in the interface between the (111) CdTe epilayer and the (100) GaAs substrate has been investigated with high resolution images taken along two different orientations. High resolution images of the CdTe epilayer which were taken along the [011¯] direction have shown asymmetrical images of Cd and Te pairs. Image calculations have been carried out in order to determine the polarity of the epilayer from these observed images.