This monograph represents a critical survey of the outstanding capabilities of X-ray diffuse scattering for the structural characterization of mesoscopic material systems. The mesoscopic regime comprises length scales ranging from a few up to some hundreds of nanometers. It is of particular relevance at semiconductor layer systems where, for example, interface roughness or low-dimensional objects such as quantum dots and quantum wires have attracted much interest. An extensive overview of the present state-of-the-art theory of X-ray diffuse scattering at mesoscopic structures is given followed by a valuable description of various experimental techniques. Selected up-to-date examples are discussed. The aim of the present book is to combine aspects of self-organized growth of mesoscopic structures with corresponding X-ray diffuse scattering experiments.