Abstract The permanence of concrete in engineering structures is jeopardized where highalkali portland cement is used with aggregates containing opal, chalcedony, tridymite, intermediate to acidic volcanic rocks, or some phyllites. First recognized in 1938, deterioration caused by cement-aggregate reaction is characterized by cracking, expansion, and decline in strength and elasticity of the concrete. Petrographic and petrologic methods have contributed significantly to the investigation of cement-aggregate reaction. Microscopic criteria serve to distinguish between this and other types of deterioration. Petrographic examination of concrete aggregates can be used to predict their potential reactivity. Physical-chemical conditions contributing to the deterioration were explored by petrologic and geochemical methods. The mechanism by which reaction between rocks and minerals and the alkalies of cement causes this destruction is explained. Essentially i t consists of osmotic pressures produced by the formation and hydration of alkalic silica gels, which arise from interaction of alkalies and certain aggregates.