The positioning of the Siluro-Devonian boundary is reviewed in the light of recent studies in both shelly and graptolitic facies in Europe and elsewhere. A case can be made for retaining the present position at the base of the Ludlow Bone Bed; but international opinion may demand a new standard section in the graptolitic facies, for example in central Europe. In this event, it is suggested that a position at the base of the Monograptus uniformis zone might prove the most widely satisfactory solution.