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Distant clusters of galaxies provide unique insight into cosmology , the formation of large scale structure , and the formation and evolution of galaxies . I will describe Chandra observations of several clusters of galaxies and show that Chandra 's arcsecond angular resolution enables studies of the physical processes in the intracluster medium in an unprecedented detail . High redshift radio galaxies are fundamentally important probes of galaxy and cluster formation . To address such questions as well as to find high-redshift clusters of galaxies , we applied optical and X-ray techniques to the areas of sky surveyed by deep ROSAT PSPC images . Their uniquely wide fields of view mean that entire clusters of galaxies can be observed simultaneously .