Ettringite is commonly formed at early ages in concrete cured under ambient conditions; this is not damaging to the concrete. But if the concrete was subjected to high temperatures during curing, ettringite formation may be delayed. Its gradual formation in the cooled, set concrete can lead to expansion and cracking in a process known as ’delayed ettringite formation’ (DEF). It can take up to 20 years for cracking to become apparent.This Information Paper, which is mainly concerned with in-situ concrete, identifies circumstances in which DEF may cause problems.