Cosmological implications of X-ray observations of clusters of galaxies

We discuss the morphology and substructure in clusters of galaxies and thei r relation to large scale structures such as filaments. As predicted by num erical simulations, the small scale structures in clusters are co-aligned w ith the much larger filamentary structures in which the clusters are embedd ed and from which they formed. For regular clusters, we review the potentia l of X-ray observations to derive gravitating masses to large radii. Using a small sample of observations, we show that the baryon fraction rises to a typical value of about 25% which, in the standard Big Bang scenario, argue s for a low value of Omega. We briefly indicate the potential for future X- ray studies of clusters with AXAF, XMM, and SRG.