High Redshift Clusters and Protoclusters

Our understanding of the cosmic history of galaxy clusters has recently been enhanced due to an extensive series of observations including faint spectroscopic data (especially those obtained at the Keck Observatory), deep optical and NIR imaging from the ground and in space, morphological data from HST, and new constraints on the evolution of the intracluster medium from ROSAT and ASCA. When such observations are applied to complete, objectively derived catalogs of clusters , our constraints on cluster formation and evolution become quite confined. A picture is emerging in which the bulk of cluster formation starts at z ≥ 2, yielding cluster potentials that are well established by z ∼ 1, and there is little substantial evolution of the cluster galaxy population since z ∼ 0.4. This review talk will summarize the current observational constraints on the properties and evolution of high redshift clusters and protoclusters and their implications.