Cosmological studies with X-ray clusters of galaxies

X-ray observations provide an ideal tool to study the basic properties of the galaxy cluster population in our Universe. These studies show that clusters form a nearly self-similar class of objects which are well characterized by the observable X-ray properties. This makes them also useful probes for the investigation of the large-scale structure of the Universe and for the test of cosmological models. Based on the ROSAT All-Sky Survey we have conducted several optical identification programs and redshift surveys of ROSAT clusters, notably: the northern NORAS Cluster Survey and the southern REFLEX Cluster Survey. We describe some of the aspects of the cluster sample construction. Based on the results of the REFLEX Survey we show results on the assessment of the large scale structure which are most consistently described in a low density cosmological model. For these large scale structure measurements the REFLEX Survey offers currently the largest available study volume.