Revolutions in Galaxy Cluster Science: Astro-H/eROSITA Synergy and Athena Outlook

eROSITA will perform the first X-ray imaging all-sky survey up to 10 keV and is expected to detect about 100,000 galaxy clusters. It is planned to be launched as the primary instrument of the Spectrum-Roentgen-Gamma mission, around the same time as Astro-H. To reach its major scientific goal -- unprecedented constraints on dark energy -- the cluster masses need to be known accurately. Astro-H will allow us for the first time to tightly constrain sources of non-thermal pressure support in galaxy clusters through its revolutionary micro-calorimeter array, and thereby to quantify one of the primary systematic uncertainties in X-ray mass measurements. The excellent synergy between eROSITA and Astro-H for clusters and cosmology will be illustrated. Moreover, at the end of the next decade, Athena is expected to be launched. Examples will be shown how Athena's combination of high throughput and excellent spectral resolution will enable breakthroughs in our understanding of cluster evolution.