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X-ray powder diffraction (XRPD) is an essential tool for the analysis of clay minerals wherever they occur and sandstones are no exception. Quantitative analysis of clay minerals in sandstones is a requirement of many investigations and the potential of XRPD in this respect is unsurpassed. The terms quantitative and semi-quantitative frequently have been used almost interchangeably and without definition. Here definitions and distinctions between these terms are offered in the context of analysis by XRPD. Application of such definitions would better enable those who use the data to identify its limitations and determine whether it is fit for the purpose, or not. The theory that underlies a large set of the published methods of quantitative analysis by XRPD is outlined. As far as possible the practical aspects of the theory have been emphasized so that the value of understanding it is apparent. Two common ways of preparing samples for quantitative analysis are described in detail, namely the analysis of orientated clay fractions and the analysis of whole-rock samples as random powders. Issues of sample preparation, measurement of peak intensities, validation and uncertainty, and lower limits of detection are discussed for each method and illustrated by examples. Additionally, the identification of the clay minerals encountered most commonly in sandstones is outlined. Studies of both orientated clay fractions and whole-rock random powders provide complementary information. For well-characterized clay minerals both methods are capable of accurate quantitative results. Nonetheless, if unknown or uncontrolled, many sample and procedural factors may intervene to place results in the semi-quantitative category.