Cluster science from ROSAT to eROSITA
  Galaxy clusters are one of the important cosmological probes to test the
consistency of the observable structure and evolution of our Universe with the
predictions of specific cosmological models. We use results from our analysis
of the X-ray flux-limited REFLEX cluster sample from the ROSAT All-Sky Survey
to illustrate the constraints on cosmological parameters that can be achieved
with this approach. The upcoming eROSITA project of the Spektrum-Roentgen-Gamma
mission will increase these capabilities by two orders of magnitude and
importantly also increase the redshift range of such studies. We use the
projected instrument performance to make predictions on the scope of the
eROSITA survey and the potential of its exploitation.
