Abstract: The application of powder X-ray diffraction in mineralogy and petrology is reviewed. It is shown that powder X-ray diffraction not only continues to play an important role in the traditional fields of phase identification, quantification and cell parameter determination, studies of isomorphism, polymorphism, polytype and order/disorder phenomena in minerals, petrofabrics and so forth, but have also found applications in the field of crystallization processes, phase transformation and surface study of minerals as well as in defects investigation and structure determination of minerals.