The stages (Brit. series) – (Tremadoc, Arenig, Llanvirn, Llandeilo, Caradoc, and Ashgill) used in the United Kingdom were found to be unsuitable for the global stratigraphy, because their sections are breaking, located in a greatly dislocated region, not all boundaries of the stratigraphic units are clear, and correlative features are insufficient. Therefore the International Commission on Stratigraphy (ICS) (Subcommission on Ordovician) decided to use in the International Stratigraphical Scale (2000) only two of seven Ordovician stages, i. e. the first one – Tremadocian, and the 4th stage – Darriwilian. The later Scale (2004) contains also the 7th stage – Hirnantian. In the Lethaia seminar S.M. Bergström et al. (2006) presented additional three new stages: the 2nd – Floian, the 5th – Sandbian, and the 6th – Katian. They are approved by the ICS under the endorsement of the International Union of Geological Sciences (IUGS). Only the third stage remained unnamed, since its lower boundary, Global Standard Section Point (GSSP), is not determined yet. The Lower Ordovician consists of two stages, Tremadocian and Floian, which are the correlated Baltic palaeobasin regional stages of Pakerort and Latorp. The Middle Ordovician has also two stages: the unnamed lower one and the upper Darriwilian, which are compared to Volkhov, Kunda, Aseri, Lasnamägi and Uhaku regional stages. The Upper Ordovician is composed of three stages: Sandbian, Katian and Hirnantian, which are correlated with the major part of the Ordovician section in the Baltic palaeobasin, i.e. Kukruse (except for its lower part), Haljala Keila, Oandu, Rakvere, Nabala, Vormsi, Pirgu and Porkuni Regional stages. The GSSP of the Ordovician stages and their sites are given in Table.