  The intrinsic transverse momentum distribution of partons in the nucleon can
be used to explain a large amount of high-$p_T$ hadron and photon production
data in high-energy nucleon-nucleon collisions at energies $\sqrt{s} \approx
20$ to 1800 GeV. However, proton-nucleus experiments at energies $\approx 30$
GeV show an extra enhancement (Cronin effect) in the yield of photons and
mesons compared to a simple extrapolation of the proton-proton data. This
enhancement is due to the effect of the dense hadronic matter encountered by
the projectile proton in the nuclear environment. We discuss the origin and the
properties of the nuclear enhancement.
