  We study the role of QCD plasma instabilities in non-equilibrium quark-gluon
plasmas. First, we argue that such instabilities must drastically modify the
"bottom-up" thermalization scenario for heavy-ion collisions. Second, we
discuss conditions for the existence of instabilities in a more general context
than previously treated in the QCD literature. We also give a thorough
qualitative review of the origin of instabilities. We discuss some mechanisms
whereby the growth of plasma instabilities saturates. Finally, we solve
explicitly for instabilities and their growth rates for two extreme cases of
anisotropic non-equilibrium plasmas that can be treated relatively simply and
analytically: $f(\p) = F(p_\perp) \delta(p_z)$ and $f(\p) = F(p_z)
\delta^{(2)}(p_\perp)$, where $f(\p)$ is the distribution of particles in
momentum space.
