Clays are the most complicated objects in quantitative mineral analysis of sedimentary rocks. Complex quantitative analysis of clays comprises four major steps: measuring bulk quantities, quantifying the mixed layering, determining the three-dimensional organization, and measuring the particle size. Computerization has resulted in major progress in all four areas during the last decade. X-ray diffraction remains the major tool of the quantitative studies of clays, supported by Fourier Transform Infrared Spectroscopy (bulk quantities), chemical analysis (bulk quantities) and electron microscopy (particle size). This contribution reviews recent developments in the techniques used for quantifying clays and their properties, and looks at the use of these quantification techniques in K-Ar dating of geological processes.