The application of oxygen and hydrogen isotope geochemistry to the study oi clays and clayey rocks has developed considerably over the past 30-40 years. Although much work remains to be completed, the knowledge of isotope fractionation between clays and water has progressed significantly for the main groups of clay minerals. in addition, the increasing number of case studies calling upon stable isotope geochemistry has provided a lot oi information on the variability of isotopic compositions of natural clays in low temperature environments, such as diagenesis and geothermal areas. The integration of isotopic data with those of mineralogy, petrography, major and trace elements geochemistry as well as site geology has resulted on many occasions, in a better characterization of the conditions of clay formation and transformation in these environments. After reviewing the basic principles of oxygen and hydrogen isotope geochemistry, we present several remarkable examples of application of this tool to the study of clay minerals in sedimentary rocks, as an illustration of the significant contribution of this discipline to the characterization oi clay genesis media. A substantial part of this article is devoted to the methods of clay separation and purification, which represent the most difficult step in many isotopic studies. Our main ambition is to provide a synthetic snapshot of the current state of knowledge in oxygen and hydrogen isotope geochemistry of clays, with the goal of introducing this discipline to students and researchers from other geoscientific fields.