XXL is an international project based around an XMM Very Large Programme surveying two 25 deg2 extragalactic fields at a depth of ~6 x 10-15 erg cm-2 s-1 in the [0.5-2] keV band for point-like sources. The XXL website is http://irfu.cea.fr/xxl. Multi-band information and spectroscopic follow-up of the X-ray sources are obtained through a number of survey programmes, summarised at http://xxlmultiwave.pbworks.com/.
*** Append here any specific observatory acknowledgements (XMM, ESO, CFHT, etc.) pertaining to the data used in the paper in question, as well as acknowledgements to the relevant funding agencies. ***