The last decade has witnessed an enormous development in the growth of epitaxial layers, hetero structures and superlattices. This progress has created new demands for the characterization of those structures. Various methods have been refined and new ones were developed with the main emphasis on non-destructive in-situ characterization. Among those, methods which rely on the interaction of electromagnetic radiation with matter are particularly valuable. In this book standard methods such as far-infrared spectroscopy, ellipsometry, Raman scattering, and high-resolution X-ray diffraction are presented, as well as new advanced techniques which provide the potential for better in-situ characterization of epitaxial structures.