  Various cosmological applications of galaxy clusters are presented. Clusters
are used to determine the baryon fraction, dark matter distribution and the
matter density of the universe. They also contain a wealth of information about
structure formation and evolution on different scales: large-scale structure,
cluster formation and also galaxy formation via the interaction of cluster
galaxies with the intra-cluster medium. In particular, the X-ray satellites
CHANDRA and XMM yield exciting new results for galaxy cluster physics and
cosmology.
