Galaxy clusters from eeHIFLUGCS, to eROSITA, to Athena

The latest results from detailed X-ray follow-up observations of large X-ray selected galaxy cluster samples are discussed, in particular from eeHIFLUGCS. An outlook is given to expected cosmological constraints from eROSITA, in particular on dark energy and neutrino masses. The possible significant role of XMM-Newton to improve on these constraints is highlighted. Finally, the expectations for Athena to discover and characterize the first galaxy groups, massive and evolved enough to contain ≫10 million Kelvin gas, around redshift 2.5 are quantified.