Chronostratigraphy is both a set of stratigraphic concepts, and a set of guiding principles by which time represented in rocks, subdivides Earth’s geological history. Chronostratigraphy is composed of a set of hierarchical stratigraphic units (i.e. System, Series, Stage, Substage and Chronozone for chronostratigraphic units) that are used to subdivide time since Earth’s formation. Most Stage divisions in the Phanerozoic rock record are now formally defined by rock characteristics (e.g. fossil content, geomagnetic polarity, isotopic events, etc.) at their lower boundary in Global Stratotype Section and Point’s (GSSP). A GSSP ’point’ is at a specific level in a sedimentary section, although Holocene GSSP’s are in ice cores and cave speleothems. For some parts of the Proterozoic and Archean, Systems are currently defined by Global Standard Stratigraphic Ages (e.g. 2500 Ma for the base of the Siderian, base Proterozoic). The application of chronostratigraphy is principally assigning rocks to chronostratigraphic divisions by a process of stratigraphic correlation, that may involve co-relating one or more sets of stratigraphic data (i.e. fossil content, chemical, magnetic, physical attributes of rocks or chronometric dating) from different section locations. Chronostratigraphic correlation is focussed on the correlation of time planes, defined by the bases of chronostratigraphic units. Chronostratigraphic divisions at a finer division than Stages, currently have no formal ratification process, so the various geological communities focussed on specific stratigraphic tools are in a process of stabilisation of the nomenclature and procedures in using these finer scale means of chronostratigraphic correlation.