Abstract The newly agreed-upon Silurian-Devonian boundary is an excellent example of a modern high precision definition of a stratotype boundary, based on biostratigraphic evidence. The Ordo- vician-Silurian boundary is being studied by a working group under two subcommittees (Ordovician and Silurian). The biostratigraphic factual material forms an excellent base for relative-time stratigraphy in the Silurian (both for the boundaries to other systems, and internally). Stratigraph- ically relevant isotope datings are few. Although many datings have been published covering the general time interval of the Silurian, only a few have been adequately tied to the fossiliferous sequence, and there is not much good evidence for the precise absolute dates of either the upper or the lower boundary. The result is that estimates of the duration of the Silurian varies with as much as 100%. The available evidence is critically examined and a dating program suggested.