Galaxy clusters are ideal probes to study the large-scale structure of our Universe and to test cosmological models. X-rays are particularly well suited to find and to study galaxy clusters. Based on the ROSAT All-Sky X-ray Survey we have obtained a comprehensive census of the galaxy cluster population in the nearby Universe. We have studied the physical structure and the statistical distribution of the structure parameters in deep X-ray observations by means of representative sub-samples drawn from this cluster population. We use a statistically complete cluster sample to assess the cosmic large-scale structure, to study superclusters, and to provide constraints on some important cosmological parameters in particular the cosmic mean matter density and the amplitude of the large-scale density fluctuations.