This paper is a revision of the standard chronostratigraphy of the Ordovician of the historical type area in England and Wales. The revision is a response to the need for more precise definitions of series and stages, especially for practical international correlation. In some cases this has entailed moving horizons for Series bases away from those in the classical sections. The scheme will be used as the standard for a forthcoming revision of the Correlation Chart of the British Ordovician System. The Ordovician is divided into five series: Tremadoc, Arenig, Llanvirn, Caradoc and Ashgill. The Llanvirn is extended to include part of the classical Llandeilo Series, which is included within it as a stage. Where stage subdivisions of standard series have not yet been defined, as in the Tremadoc, they are proposed in this paper. However, Bancroft’s fine stadial divisions of the Caradoc, which have been criticized for their brevity and local utility, are combined into four new stages, which will have wider application. The former Caradoc subdivisions are retained as substages for the purposes of regional correlation.