  Multi-particle production in QCD is dominated by higher twist contributions.
The operator product expansion is not very effective here because the number of
relevant operators grow rapidly with increasing twist. The Color Glass
Condensate (CGC) provides a framework in QCD to systematically discuss
"classical" (multiple scattering) and "quantum" evolution (shadowing) effects
in multi-particle production. The apparently insuperable problem of
nucleus-nucleus scattering in QCD simplifies greatly in the CGC. A few examples
are discussed with emphasis on open problems.
