The XMM-Newton Serendipitous Survey has been detecting X-ray sources at a rate of 50,000 per year. As part of the XMM serendipitous source identification programme (XID) of the XMM Science Survey Center (Watson et al. 2001), we have been obtaining optical multi-colour CCD imaging data for a large number of XMM fields. The multi-colour CCD imaging data for high Galactic latitude fields is currently being obtained with the wide field CCD mosaic cameras at the 2.5m Isaac Newton Telescope in La Palma and the 2.2m MPG/ESO Telescope in Chile. The scientific goals are: to provide optical/IR counterparts for the need of the XID spectroscopic identification programme (e.g. Barcons et al. 2002) and photometric identification of XMM sources for which spectroscopic data are not available; to explore in a statistical manner various topics such as hard X-ray sources, the CXB, objects at high redshifts, and AGN evolution.