Clusters of galaxies, as the largest, well-defined astronomical objects, are interesting giant astrophysical laboratories and important probes for the large scale structure of the Universe. In X-rays they can be observed and studied as whole entities. In particular, recent X-ray observations with the ROSAT Observatory have advanced our understanding of the dynamical structure and the mass distribution in clusters. The Sunyaev-Zel’dovich effect makes use of the intracluster plasma to determine cosmological distances independent of the traditional cosmological distance ladder.