  Fluid dynamical models preceded the first heavy ion accelerator experiments,
and led to the main trend of this research since then. In recent years fluid
dynamical processes became a dominant direction of research in high energy
heavy ion reactions. The Quark-gluon Plasma formed in these reactions has low
viscosity, which leads to significant fluctuations and turbulent instabilities.
One has to study and separate these two effects, but this is not done yet in a
systematic way. Here we present a few selected points of the early
developments, the most interesting collective flow instabilities, their
origins, their possible ways of detection and separation form random
fluctuations arising from different origins, among these the most studied is
the randomness of the initial configuration in the transverse plane.

Nuclear Theory