X-ray powder diffraction methods have been more and more extensively used, in the last twenty years, in the characterization of different crystalline and non-crystalline materials of archaeological, historical, artistic interest. The present paper aims to discuss the use of these methods in studying different aspects related to the Cultural Heritage conservation. This purpose has been achieved by examining the results obtained during the studies carried out by the author and his collaborators on a number of objects formed by different materials, such as cellulose, paper, wood, papyrus, archaeological glasses, mosaics (non crystalline or only partially crystalline materials), ceramic, metals, their corrosion products, slags, stones, mortars, pigments (partially or almost completely crystalline materials).