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In its far too brief life, the Japanese X-ray satellite Hitomi gave us a taste of things to come. With its exquisite spectral resolution, it directly measured the Doppler line broadening and centroid shifts of the lines from H- and He-like ions of Fe in the core of the Perseus cluster. The observed velocities were of order 100-200 km/s, providing the first direct measurements of the streaming and random gas motions in the central regions of a cluster ICM, due to both gravitational and AGN energy inputs. They also confirm that the ICM is still very close to hydrostatic equilibrium despite these exciting signposts of ICM activity.

Astronomers are quite excited about what's coming next. On the X-ray side, eROSITA, a joint German-Russian venture is projected to detect the ICM in 50-100 thousand clusters, dramatically expanding our capacity for statistical and cosmological studies. The exquisite spectral resolution and sensitivity of XRISM, a joint Japanese/U.S. mission, will map out the details of velocity structures in the ICM. Further down the road, the European satellite ATHENA will allow us to study the history of how the ICM is heated and chemically evolves over cosmic times, while NASA is considering the Lynx mission, which would be able to probe the diffuse medium in cluster-feeding filaments down to the astonishing low value of ∼ 7 atoms/m3.

Complementing these are a new generation of radio telescopes and surveys which will probe the cosmic rays and magnetic fields. At low frequencies, the LOFAR Two Meter Survey and the Murchison Widefield Array have begun to reveal the extensive cluster synchrotron structures that have faded at higher frequencies due to radiative losses. Polarization at low frequencies, including new capabilities at the VLA, will provide detailed probes of magnetic field irregularities in the ICM. And large sky coverage polarization surveys such as the Polarization Sky Survey of the Universe's Magnetism on the Australian SKA Pathfinder telescope and the Very Large Array Sky Survey will allow statistical studies to connect the magnetic structures in the ICM with other cluster physical properties. Even more is on the horizon.

So clusters will continue to evolve, our instrumentation will get ever more powerful, our simulations increasingly sophisticated, and the weather outlook for the ICM is stormy indeed!

Thanks to many colleagues across the spectrum, for their most wise counsel, and for support from the NSF through grant AST17-14205. And apologies to everyone for the lack of references, as per the PT style.

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