The dominance of Portland cement concrete as the world's pre-eminent structural material developed during the nineteenth century and continued throughout the twentieth century and retains that position today.However, the material had a 'guilty secret' that was only revealed by research in the USA during the 1930s. In some circumstances, an expansive reaction can gradually occur between aggregate constituents and alkaline hydroxides from the cement, causing damage to the hardened concrete within structures.This has become known as 'alkali–aggregate reaction' (AAR), or most commonly and more specifically, 'alkali–silica reaction' (ASR). This AAR family of mechanisms is by no means the most frequently encountered threat to concrete durability or the serviceability of structures, but it can be a serious issue when it occurs.It has continued to fascinate concrete research scientists and engineers ever since its first published description, some 76 years ago.Getting to grips with AARAfter graduating as a geologist in London in 1972, I was introduced to the 'guilty secret' of AAR as I commenced PhD research into the chemical instability of some concrete in the UK. This project covered a range of mechanisms, but unbeknown to me at the time, it would be AAR that would come to dominate my professional life in construction materials consultancy.Over more than 40 years, I have been privileged to witness practising engineers and applied scientists getting to grips with AAR worldwide. I believe we now understand the several reactions, including how to recognise them in existing structures and manage the situation, but crucially also including how to determine any possible AAR potential in new works and take effective precautionary measures.However, challenges remain, especially as appreciation of the potential threat from AAR is not universally or equally appreciated everywhere in the world (Sims and Poole, 2016), and we are still finding types of structure that behave exceptionally. Overall, the prospects are encouraging for both a declining incidence of AAR damage and successful approaches to management and/or repair of affected cases.Conferences and journalsSince 1974, the progress of understanding AAR has been periodically reviewed by the International Conference on AAR (ICAAR). The 15th ICAAR takes place in São Paolo, Brazil in July 2016. To coincide with the conference, the Institution of Civil Engineers has commissioned AAR experts to write 14 papers for two themed issues of its Construction Materials journal on the topic. The first issue, 169 CM3, was published in June 2016 and the second, 169 CM4, will come out in August 2016.Indeed, the opening paper in the first issue (Andiç-Cakir et al., 2016) provides an introduction to AAR in the context of the series of ICAARs, from the first small meeting in Denmark in 1974 through to the much larger gatherings in more recent years. An indication of the rapid global spread of the recognition of AAR is illustrated by an interesting world map that is doubtlessly already out of date, while examples are provided of some key AAR developments.Each of the papers in the two issues is deliberately concise to achieve a compact overview of a complex subject, but also contains references to key publications in which more exhaustive information may be found, when required. Further progress may be expected at the 15th ICAAR in Brazil and those who cannot attend may wish to obtain a copy of the conference proceedings.