Internal interfaces in materials like e.g. semiconductor heterostructures get more and more interest not only under aspects of basic research but as well under aspects of new electronic and optoelectronic devices. The interface properties often govern the device performance. Thus, the evaluation of individual heterointerfaces with respect to chemical composition and crystal structure requires characterisation techniques which offer the necessary high spatial resolution. The fine focused electron probe (< 0.3 nm at 100 keV) in a field-emission STEM (Scanning Transmission Electron Microscope) allows the application of special imaging and analytical techniques to cross-sectional specimens of semiconductor heterostructures. Qualitative information on the chemical composition is provided by atomic number (Z) contrast imaging with atomic resolution. The same fine probe can be used to analyse subnanometer areas by both spectroscopic and diffraction techniques. Quantitative compositional information is provided by electron energy-loss spectroscopy (EELS) which allows the detection of concentrations of specific elements.