Most events characterising stratigraphic boundaries span an interval of time. The boundaries are often diachronous and they may be affected by environmental conditions. Therefore, the only accurate way to define a boundary is to select a boundary stratotype which records as many paleontological, geophysical and geochemical events as possible. Conventional stratigraphic boundaries are commonly difficult to identify precisely outside the stratotype area. This is why it is necessary to use first and last appearances of diagnostic features which occur below and above the conventional boundary. In this point of view the Eocene-Oligocene boundary is particularly significant.