One of the most classical techniques for structure analysis is X-ray diffraction (XRD). Many variants of diffraction experiments have been developed. They can for example be classified into techniques for the examination of X-ray scattering in the wide angle or small angle range. Furthermore, different X-ray sources can be used, with synchrotron radiation opening an own complex field of research within the diffraction experiments. Rather than to give a summary on the fundamentals of X-ray diffraction and the variety of different existing methods, the aim of this chapter is to provide a review on what kind of information common X-ray diffraction methods, initially developed for extended crystalline solids, can reveal in the case of nanocrystalline materials and also soft matter such as polymer. Apart from the identification of crystalline phases, X-ray diffraction is in particular used to determine the size of nanoparticles, and special interest will be devoted to that task here. The main part of this chapter will be devoted to X-ray diffraction in the wide angle range. A brief overview on small angle X-ray scattering (SAXS) will be given as well.