The physics regulating the dense gas at the centers of galaxy clusters
is far from being fully understood. Cooling due to X-ray emission and
heating, e.g., due to AGN, seem to be tightly coupled. Here, I'll
report on our recent results using X-ray and radio data of a complete
sample of nearby galaxy clusters. Moreover, cluster outskirts up to
and beyond the virial radius have recently become accessible to X-ray
observations. I'll describe our latest gas temperature profile
measurements and also the combination of X-ray with Sunyaev-Zeldovich
data. Furthermore, new substructure measurements with X-ray and
optical velocity dispersion data are described. Also, I'll discuss the
status of our analaysis of a complete galaxy group sample and our weak
lensing follow-up of a complete sample of distant galaxy clusters.
Finally, the prospects of using galaxy clusters as cosmological probes
with the upcoming eROSITA mission are described.