After the first definitions of Quaternary, this term has been widely discussed, starting a still open debate mainly focussed in the definition and setting of its lower boundary. The first steps were taken during the 18th International Geological Congress (London, 1948), where it was stated that the boundary should be based in a faunal change within a marine sequence in Italy, indicating the first clumatic deterioration in the Mediterranean. After almost four decades of discussions, during the 27th International Geological Congress (Moscow, 1984), the Vrica section was accepted as Global Stratotype Section and Point (GSSP) for the definition of the P/P Boundary as proposed by Aguirre and Pasini, (1985). The official P/P Boundary is thus located in the Vrica section (Italy) and currently situated slightly below the top of Olduvai, recently recalibrated at 1.8 My and coincident with I.S. 64. The scientific and technologic advances have opened a new and wide view on this concern, increasing the quality and quantity of reference data. In this sense many voices have risen in favour of lowering this P/P Boundary to the Gauss/Matuyama boundary (ca. 2.6 My), coincident with the first appearance of Neogloboquadrina atlantica in the Mediterranean (considered as the true first northern guest) during I.S. 104. During the IVX INQUA Congress (Berlin, 1995) a session was devoted to the P/P Boundary, and the results were included in a special issue of Quaternary International (vol. 40, 1007), a review of which is included here. More recently, the Subcommission on Quaternary Stratigraphy of the I.C.S., after a postal ballot, resolved that this boundary should be lowered to the vicinity of the Gauss/Matuyama paleomagnetic reversal. Nevertheless this debate is still open. This paper also includes a review of the state of the art of the Plio-Pleistocene boundary in Spain.
