Approximately forty years ago, it was realised that the generation of high flame speeds would result in damaging pressures being produced in vapour cloud explosions (VCEs). At the time, however, it was not clear how high flame speeds could be generated in VCE. Research in the 1970s and 1980s showed that if a vapour cloud engulfed a region of congested process pipework, the flame could accelerate to levels where damaging pressures were produced. Since that time, the accepted basis for assessing VCE hazards has been to consider congested process regions as potential locations of deflagrations, with pressure reducing as the explosion propagated away from the region. However, this was not sufficient to explain the evidence found following the Buncefield explosion in the UK (2005). The research conducted following Buncefield, the most extensive for any VCE, led to the conclusion that the incident involved a transition from a deflagration to a detonation (DDT). In reaching this conclusion, several possibilities were considered and dismissed, including a speculative explosion mechanism not previously observed that involved ignition by radiation of dust particles ahead of the flame. Taken in combination with more recent experimental studies, they indicate that DDT was not just an explanation for Buncefield, but likely has a much wider relevance. The evidence obtained from Buncefield and subsequent research supporting DDT will be reviewed.