The Paleocene / Eocene boundary has been studied for the last seven years by the Spanish working group of ICCP project 308. Many sections have been sampled and studied, concluding that the most suitable sections to define the PaieoceneiEocene boundary are the following: Alamedilla, Campo, Caravaca, Ermua, Tremp and Zumaya. High resolution and integrated stratigraphical studies have been accomplished and the correlation with the stratotypes of the European marine classical stages has been established. The mass extinction horizon of the bathyal and abyssal small benthic foraminifera constitutes the most relevant event near the base of the Ypresian stratotype, which is the standard stage for the early Eocene. This event coincides with other conspicuous bioshatigraphical and geochemical changes, that can be clearly identified in the best Spanish sections. The integrated stratigraphy across the Paleocene / Eocene boundary presented in this paper is the most complete model known so far. Besides, the excellent characteristics of several of these sections and the high resolution studies accomplished, enable to propose the definition of the Paleocene / Eocene boundary stratotype in any of them, concluding that the Zumaya section is the most suitable one