Diagnosis of damage to concrete in structures caused by Alkali-Aggregate Reaction (AAR) often involves complex processes because of the resemblance of the external characteristics of the damage with those caused by other actions, such as e.g. frost attack or shrinkage. The wide variety of aggregate types used in concrete worldwide, the different reactivity and proportions of the reactive constituents in the aggregates involved and the wide range factors that influence the reaction result in wide variability found in site samples. This makes diagnosis difficult and has sometimes led to subjective or even erroneous interpretations and conclusions. Furthermore, there has frequently been a lack of agreement amongst experts on how to approach the issues of diagnosis and prognosis of AAR damage in structures.