Abstract: Fabric (texture and structure) and mineralogic composition determine the response of clays and shales to events occurring during construction and operation of engineering works. A new system of classification of the fabric of earth materials is proposed. Characteristic mineralogic composition of clays and shales, especially in the western United States, is described. Fabric and composition are correlated with soil mechanics properties and engineering performance. Needed research on clays and shales as a basis for design, construction, and maintenance of engineering structures is emphasized.