Recent large-scale experiments have indicated that scale and geometric effects can strongly influence both the ability of a fuel-air mixture to propagate or sustain a detonation, and the probability that a deflagration will undergo a transition to detonation. The data show that many past concepts concerning detonations, which were derived from small-scale experiments, should be revised. In particular, the occurrence of detonations may not be as unlikely as previously considered, especially in large-scale industrial environments in which obstacles are present. Some of the important recent experimental research is reviewed and some qualitative ideas concerning the interpretation of the experiments and their implications for improving safety are provided.