  Modern galaxy cluster science is a multi-wavelength endeavor with
cornerstones provided by X-ray, optical/IR, mm, and radio measurements. In
combination, these observations enable the construction of large, clean,
complete cluster catalogs, and provide precise redshifts and robust mass
calibration. The complementary nature of these multi-wavelength data
dramatically reduces the impact of systematic effects that limit the utility of
measurements made in any single waveband. The future of multi-wavelength
cluster science is compelling, with cluster catalogs set to expand by orders of
magnitude in size, and extend, for the first time, into the high-redshift
regime where massive, virialized structures first formed. Unlocking
astrophysical and cosmological insight from the coming catalogs will require
new observing facilities that combine high spatial and spectral resolution with
large collecting areas, as well as concurrent advances in simulation modeling
campaigns. Together, future multi-wavelength observations will resolve the
thermodynamic structure in and around the first groups and clusters,
distinguishing the signals from active and star-forming galaxies, and unveiling
the interrelated stories of galaxy evolution and structure formation during the
epoch of peak cosmic activity.
