The correct identification of potentially alkali–silica reactive aggregates is important for the prevention of alkali–silica reaction (ASR) in concrete. Although a number of standards for assessment of concrete aggregate by petrography are available, distinction of potentially deleterious from innocuous rock types can be problematic. The application of geological nomenclature alone is insufficient, as the geological history and hence mineralogical texture of a given rock type may strongly influence its performance in concrete. One of the goals of RILEM TC 219-ACS is to develop a worldwide photo atlas as a guide for petrographers in the identification of the mineral compositions and textures that are characteristic of alkali-reactive rocks. The atlas is based on micrographs of rock types recognized as potentially deleterious by field performance and/or laboratory expansion testing. It is intended to serve as an independent reference work and aims to unify rock terminology so as to improve the petrographic characterization of aggregates. The mission of RILEM, the International Union of Laboratories and Experts in Construction Materials, Systems and Structures (Réunion Internationale des Laboratoires et Experts des Matériaux), is to advance scientific knowledge in the field of construction. Its Technical Committee TC 219-ACS focuses on alkali reactions and their effects on concrete.