This chapter provides several examples that illustrate the role of petrography and the need for a new direction in aggregate research. It discusses the concept of durability and reviews the information laboratory testing methods of geomaterials. A general introduction, describing the special nature of geomaterials with their varying mineralogical composition and texture, is presented. The geological construction materials that are commonly used in civil engineering construction are mainly natural soil aggregates and crushed rock or rock blocks. The microscopic structure, just like the internal structure of other materials such as steel or concrete, determines to a large extent the mechanical behaviour of the geological materials. The ideal approach to the evaluation of geomaterials would make use of this aspect and incorporate probabilities, based on the variability of observations and test results, rather than fixed values. Weathering can lead to considerable variation of the geotechnical properties within a rock unit.@en