Alkali-silica reaction is a mechanism of deterioration of concrete structures which is associated with the composition of the rocks used as aggregates. The assessment of aggregates to evaluate their potential reactivity to alkalis is made both by visual methods and laboratory expansion tests. There are a number of national standards regarding the petrographic examination of new aggregates but the identication of the features that are related with the reactivity is not always easy. Supplementary to the recommendation of the petrographic method, RILEM has developed in recent years a worldwide petrographic atlas aiming at providing information regarding the features typical of each rock type that might be involved in alkali-silica reactions. In this work, a presentation is made on the results of the petrographic study of about 70 samples received from 25 countries in all continents. The samples include rock fragments, sand and gravel, concrete from structures and from prims subjected to expansion tests.