Hard Scattering in a Nuclear Environment: Farewell to Linear   $k_{\perp}$-Factorization
  We discuss a dramatic change brought into the pQCD description of hard processes in a nuclear environment by a large thickness of heavy nuclei. It breaks the familiar linear $k_{\perp}$-factorization which must be replaced by a new concept of the nonlinear $k_{\perp}$-factorization introduced in \cite{Nonlinear}.We demonstrate the salient features of nonlinear $k_{\perp}$-factorization on several examples from hard dijet production in DIS off heavy nuclei to single-jet to dijet production in hadron-nucleus collisions >. We also comment briefly on the non-linear BFKL evolution for gluon density of nuclei. 
N.N. Nikolaev (Institut f. Kernphysik, Forschungszentrum J\"ulich,   Germany and L.D.Landau Institute, Chernogolovka, Russia...