The advances in cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/XSTS) are reviewed with emphasis on the structural and spectroscopic studies of semiconductor heterostructures on an atomic or nanometer scale. The unparalleled chemical and electronic sensitivities as well as spatial resolution of XSTM/XSTS are addressed. Examples shown here include recent results of cation/anion selective imaging, observations of individual impurities, interface roughness and asymmetry, band

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... ffset determination, and nanometer-resolution band mapping in prototype (AlGa)As / GaAs low-dimensional multilayered systems. Current developments in use of XSTM/ XSTS combined with other techniques for the exploitation of optical properties of semiconductor heterostructures are also discussed.