Galaxy clusters have long served as powerful probes of both cosmology and 
galaxy evolution, providing insights into the role of environment on galaxy 
evolution, evidence for dark matter, and constraints on the dark matter and 
dark energy content of the universe. The high-redshift frontier of galaxy 
cluster studies now lies at the era of cluster formation, during which the 
first massive galaxy clusters assemble. It is in this era that the present 
day star-formation density relation is established, and during which 
properties of the galaxy cluster population are strongly sensitive 
to cosmology. 
In this talk I will present results from high-redshift cluster searches 
with the Spitzer and WISE telescopes, which are helping to open up this 
redshift window for detailed study of the cluster galaxy population. I will 
show that by z=1.5 we are finally witnessing significant galaxy assembly 
and in situ star formation.  Additionally, I will demonstrate how these 
programs are pushing the extremes in redshift and mass for detection of 
galaxy clusters, and discuss potential cosmological implications.