This chapter proceeds to the next step after collecting a quality powder diffraction pattern: the interpretation of the diffraction data. The first stage involves preliminary data treatment, qualitative analysis (often referred to as phase identification), and semi-quantitative or quantitative analysis using database search–match, all performed and illustrated with a variety of crystallographic databases and software packages. Finally, an introduction to advanced phase quantification using the Rietveld method and a review of methods for amorphous content determination are presented.