  An event generator for nuclear collisions is a microscopic model, obtained
from extrapolating elementary interactions -- as electron-positron
annihilation, deep inelastic scattering, and proton-proton interactions --
towards proton-nucleus and nucleus-nucleus scattering, by using Monte Carlo
techniques.
  In this paper, we will discuss the physical concepts behind such event
generators. We first present some qualitative discussion of nuclear scattering,
before discussing particle production and strings. We then discuss the parton
model, and finally multiple scattering theory.
