The Reynolds Cup (RC) is a unique round-robin competition that was established by The Clay Minerals Society in 2000 to assess the level of precision and accuracy that is attainable for the mineralogical analysis of a wide range of complex clay-rich materials. Although the Reynolds Cup round-robin allows any possible analysis methods, X-ray diffraction (XRD) is by far the most frequently used technique. It is not only used to identify components, but also for quantitative phase analysis (QPA). QPA means determination of the relative concentrations of the coexisting phases in a mixture, commonly as a weight percent (wt.%) or mass fraction. Several approaches allow a quantitative determination of mineral contents, such as the Rietveld method (Rietveld, 1967). The successful application of the Rietveld method for QPA requires that all components are correctly identified and that the component diffraction patterns are appropriately described, which is preferably based on structure. In addition, the quality of a Rietveld quantification also depends on suitable sample preparation and measurement conditions, as well as a correct description of instrument configurations. Results from all previous Reynolds Cup contests show that a successful quantification depends strongly on the skill of users. Although the refinement procedure itself is automatic and, therefore, user independent, the results are strongly influenced by the structural models and refinable parameters that are selected and on the limitations of those parameters. Selected examples for the successful application of Rietveld refinement as well as the limitations of the method will be discussed in this article. The goal of the present work was to demonstrate that the Rietveld method is in principle capable of quantifying all Reynolds Cup samples with a high degree of accuracy, but sample specific difficulties and analysts’ inexperience may impede successful application. Incorrect results are often not indicated simply by low residuals or good fits. All refinement results should be validated and corrected using supplementary techniques, even if the results appear acceptable.