  We review basic theoretical concepts to describe nuclear collisions at
ultrarelativistic energies. We discuss relativistic strings, Gribov--Regge
theory (GRT) of hadronic interactions and string models based on GRT, and
finally generalizations to nucleus--nucleus scattering.
  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Invited Lecture at the `` International School--Workshop on Relativistic Heavy
Ion Physics'', Prague, Czech Republic, Sept. 19-23, 1994
