The Evolution of Galaxy Clusters Across Cosmic Time
  The large scale structure of the present Universe is determined by the growth
of dark matter density fluctuations and by the dynamical action of dark energy
and dark matter. While much progress has been made in recent years in
constraining the cosmological parameters, and in reconstructing the evolution
in the large--scale structure of the dark matter distribution, we still lack an
understanding of the evolution of the baryonic component of the Universe.
  Located at nodes of the cosmic web, clusters of galaxies are the largest
collapsed structures in the Universe with total masses up to 10$^{15}$
M$_{\sun}$. Over 80% of their mass resides in the form of dark matter. The
remaining mass is composed of baryons, most of which (about 85%) is a diffuse,
hot plasma that radiates primarily in X-rays. X-ray observations of the
evolving cluster population provide a unique opportunity to address such open
and fundamental questions as: How do hot diffuse baryons dynamically evolve in
dark matter potentials? How and when was the excess energy which we observe in
the intergalactic medium generated? What is the cosmic history of heavy-element
production and circulation?
  Our current knowledge comes primarily from detailed studies of clusters in
the relatively nearby Universe (z$<$0.5). Major advances will come from high
throughput, high spectral and spatial resolution X-ray observations that
measure the thermodynamical properties and metal content of the first low mass
clusters emerging at z $\sim$ 2 and directly trace their evolution into today's
massive clusters.
