Distinctive tephra layers constitute important time-parallel markers, which if widespread, offer the potential for reliable correlation over long distances. Confident correlations require a multiple criterion approach to tephra characterisation and equivalence of samples should only be considered firmly established if their stratigraphic, palaeontologic, palaeomagnetic, and radiometric age relations are compatible, and the physicochemical properties of their glass shards and phenocrysts agree. The strong susceptibility of tephra to reworking further argues for use of several stratigraphic controls in order to safeguard against gross errors.