Abstract Two remarkable events in the history of life on the Earth occur during the Ordovician Period (486.9–443.1 Ma). The first is an exceptionally rapid and sustained radiation of marine life known as the ‘Great Ordovician Biodiversification Event’ (GOBE), and the second is a catastrophic Late Ordovician mass extinction (LOME). Understanding the duration, rate and magnitude of these events requires an increasingly precise global correlation framework. In this chapter we review the major subdivisions of the Ordovician System, their Global Stratotype Section and Points, and the chronostratigraphic levels that define their bases. We also present a detailed set of correlation charts that illustrate the relationships between most of the regional graptolite, conodont and chitinozoan successions across the world.