The Working Group (WG) on the Paleocene/Eocene (P/E) boundary will soon have to submit a formal proposal regarding the selection of a GSSP for the boundary. Before it proceeds, a number of critical points must be considered. First, there has been dual definition of the Paleocene/Eocene boundary so that even today the boundary is placed at two different lithostratigraphic horizons by marine and vertebrate paleontologists. The former use the base of the Ypresian Stage = Mont Heribu Member in the Belgium Basin whereas the latter use the Conglomerat de Meudon believed until recently to lie at the base of the Sparnacian Argile plastique of the Paris Basin. A historical review of the definitions of the terms that have become accepted as standard chronostratigraphic units help to explain how this dual definition arose and to show that the definition accepted by most vertebrate paleontologists is spurious. The current placement of the Paleocene/Eocene boundary, i.e., defined by the base of Ypresian Stage (Ieper Clay), conforms to the definition of the Paleocene by Schimper in 1874. Second, the WG must determine whose basic chronostratigraphic procedures to follow. There has been in recent years a profound shift in chronostratigraphic procedures so that the fundamental principles recognized by the International Subcommission on Stratigraphic classification [ISSC, Hedberg, Ed, 1976; Salvador, Ed., 1994] are difficult to reconcile with the rules published by the International Commission on Stratigraphy (ICS) in recent years, based on the precept that "correlation precedes definition". For this reason the WG finds itself in a dilemma, and must weigh the risks associated with the attractive solution that would consist in defining the P/E boundary by a lithostratigraphic level where the late Paleocene carbon isotope excursion (CIE) is recorded. Four possible options that may satisfy Hedberg's principle of chronostratigraphy and/or the need for recognizing globally the Paleocene/Eocene boundary are presented. Whichever option is followed, it is important to remember that chronostratigraphy must remain objective and as arbitrary as possible, and thus independent of any aspect of Earth history, either paleobiologic, tectonic or climatic. In addition, in making a proposal for the definition of the P/E boundary, the WG must ensure that its choice will not contribute to a destabilisation of the current chronostratigraphic framework.

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