This article surveys the debate on the origin of periphrastic do, with particular attention to the hypothesis that Celtic languages might have exerted some influence. With respect to the facts, it is argued that there are various types of Celtic hypotheses and that one type is sensible, though unlikely to be proven and even less likely to be proven to be the only relevant factor. With respect to the debate itself, it is shown that a Celtic origin hypothesis is accepted more widely among non-British scholars, and we speculate why that might be the case.