A world-wide search for stratigraphic sections exposing the base of the Nemagraptus gracilis Biozone, the level selected to define the base of the global Upper Ordovician Series, resulted in the recognition of three potential boundary stratotypes, namely the Fågelsång, Calera, and Dawangou sections, all of which have been restudied during the last four years. These sections fulfill most of the requirements for a GSSP, and their biozone boundaries can be correlated precisely with each other. Based on an assessment of all information now available, it is concluded that the Fågelsång section is slightly superior to the others as a global boundary stratotype. It has excellent and well-known graptolite and conodont biostratigraphy in a stratigraphically continuous succession of uniform lithology, is readily accessible, and the key interval is not affected by faulting. The boundary interval is exposed in two natural outcrops that have not changed substantially for more than a century, and the same interval is also present in two drill-cores from the Fågelsång region. Although having some drawbacks, the Calera and Dawangou boundary sections are likewise excellent in most significant respects and they are proposed to be selected as auxiliary stratotypes.