High-redshift X-ray clusters

Despite their extreme importance for cosmology, the number of known high redshift clusters is rather small. 4 few clusters at high redshift have been discovered through their X-ray emission. Follow-up observations of these clusters with X-ray telescopes, and deep imaging from the ground and from HST, have shown that massive bound structures at high redshift are not as rare as was once believed. Weak lensing techniques have provided masses of the order of 10(14) - 10(15) M.. I will present results obtained on high-z, massive clusters extracted from the Einstein Medium Survey and the ROSAT NEP survey. The study of these clusters can provide stringent constraints on theories of large-scale structure formation.