The phenomenon of expansion in concretes which have been exposed to elevated temperatures during curing and subsequently to moisture is discussed; in particular, the relationship of this expansion to the formation of ettringite after initial curing (delayed ettringite formation). Evidence is also presented for the possible implication of the C-S-H gel in the expansion process. The avoidance of this type of expansion in field concrete is also discussed. The data show that expansion occurs only when the concrete temperature during hydration exceeds 65°C and that the proportion of cements that expand under laboratory testing conditions increases as the temperature of exposure increases. Tests on limited numbers of cements have indicated that some factors in the cement that appear to influence the tendency of the concrete to expand after elevated temperature curing (such as, alkalis content, fineness, SO3), but there is no general relationship between these factors and the degree of expansion valid across the range of commercial cements. This means that the selection of a cement within certain limits will not guarantee the avoidance of expansion if the concrete is exposed to a high enough temperature during curing.