We consider collisions of hadrons/nuclei at high energies. We show that at high energy a hadron/nucleus wave function contains many gluons which carry a small fraction x of the hadron's energy. This high occupancy state is approximated as a classical field and gives the dominant contribution to hadronic cross sections at high energy. A Wilsonian Renormalization Group approach is developed and used to investigate the energy dependence of QCD observables in this limit. Connections to interactions of ultra-high energy neutrinos and cosmic ray physics will be pointed out and applications to particle production in currently operating colliders such as RHIC and the LHC will be discussed.