The XMM Large Scale Structure survey (XMM-LSS) is a medium deep large area X-ray survey. Its goal is to extend large scale structure investigations attempted using ROSAT cluster samples to two redshift bins between 0< z< 1 while maintaining the precision of earlier studies. Two main goals have constrained the survey design: the evolutionary study of the cluster–cluster correlation function and of the cluster number density. The adopted observing configuration consists of an equatorial mosaic of 10 ks pointings, separated by and covering 8� 8, giving a pointsource sensitivity of in the 0.5–2 keV band. This will yield more than 800 clusters of galaxies and a sample of X-ray AGN with a space density of about 300 deg− 2. We present the expected cosmological implications of the survey in the context of ΛCDM models and cluster evolution. We give an overview of the first observational results.