X-ray Surveys of Distant Galaxy Clusters
  I review recent observational progress in the search for and study of distant galaxy clusters in the X-ray band, with particular emphasis on the evolution of the abundance of X-ray clusters out to z~1. Several on-going deep X-ray surveys have led to the discovery of a sizeable population of clusters at z>0.5 and have the sensitivity to detect clusters beyond redshift one. These surveys have significantly improved our understanding of cluster evolution by showing that the bulk of the population of galaxy clusters is not evolving significantly since at least z~=0.8, with some evolution limited to only the most luminous, presumably most massive systems. Thus far, a well defined sample of very high redshift (z>~1) clusters has been difficult to assemble and represents one of the most challenging observational tasks for the years to come. 
Piero Rosati