  This contribution illustrates the study of galaxy clusters as astrophysical
laboratories as well as probes for the large-scale structure of the Universe.
Using the REFLEX Cluster Survey, the measurement of the statistics of the
large-scale structure on scales up to 500 $h^{-1}$ Mpc is illustrated. The
results clearly favour a low density Universe. Clusters constitute, in
addition, well defined astrophysical laboratory environments in which some very
interesting large-scale phenomena can be studied. As an illustration we show
some spectacular new XMM X-ray spectroscopic results on the thermal structure
of cooling flows and the interaction effects of AGN with this hot intracluster
medium. The X-ray observations with XMM-Newton show a lack of spectral evidence
for large amounts of cooling and condensing gas in the centers of galaxy
clusters believed to harbour strong cooling flows. To explain these findings we
consider the heating of the core regions of clusters by jets from a central
AGN. We find that the power output the AGN jets is well sufficient. The
requirements such a heating model has to fulfill are explored and we find a
very promising scenario of self-regulated Bondi accretion of the central black
hole.
