One of the first major studies of weathering and soil formation was made by Harrison (1933) who used thin sections in association with other procedures to study the transformation of minerals in different kinds of rock under the tropical conditions of Guyana. However, Kubiena (1938) is regarded as pioneering thin section studies of soils and during the last two decades there has been a rapid increase in the number of publications devoted almost exclusively to the study of soils in thin sections. In addition to the rather straightforward examinations with the polarizing microscope, thin section techniques are being linked with X-ray diffraction, X-ray microprobe, transmission and scanning electron microscopy, microbiological and other procedures to obtain a fuller insight into the composition and genesis of soils. Thus the study of thin sections of soils is now a major pedological technique for investigating small details in the nature, type and degree of organization of the soil fabric and structure. Thin sections reveal that particles of various sizes and composition react differently to pedological processes and become weathered or organized to form many specific patterns. This book is an attempt to give a comprehensive treatment of thin section studies of soils. Although primarily about the study of thin sections with optical microscopes a few transmission and scanning electron photomicro­ graphs are included to confirm the inferences based upon the studies made with the optical microscope.