In the last twenty-five years, relevant theoretical, methodological and experimental advances have been made in the development and application of the X-ray powder diffraction (XRPD) method. In particular, attention has been devoted to the interpretation of XRPD data. The XRPD approach is used currently in mineralogical as well as in many other scientific fields (solid-state chemistry, pharmacology, materials science, etc.) to address a wide range of scientific purposes: qualitative analysis for the identification of the crystalline phases constituting a powder sample; quantitative analysis for estimating the weight fraction of each phase in a mixture; structure solution; microstructural analysis for the inspection of crystalline domain size effects and lattice defects; investigation of highly complex materials: compounds with incommensurate structures, nanoparticles, amorphous materials; studies at non-ambient conditions, in situ, time-resolved and in operando for the description of thermal or compressional behaviour, phase stability and structural evolution. The aim of this chapter is to provide an overview of some basic principles and significant aspects of the XRPD method and examples of its applications to mineralogical problems