  At asymptotically high energies, thermalization in heavy ion collisions can
be described via weak-coupling QCD. We present a complete treatment of how
thermalization proceeds, at the parametric weak-coupling level. We show that
plasma instabilities dominate the dynamics, from immediately after the
collision until well after the plasma becomes nearly in equilibrium. Initially
they drive the system close to isotropy, but Bjorken expansion and increasing
diluteness makes the system again become more anisotropic. At time \tau ~
\alpha^(-12/5) Q^(-1) the dynamics become dominated by a nearly-thermal bath;
and at time \tau ~ \alpha^(-5/2) Q^(-1)$ the bath comes to dominate the energy
density, completing thermalization. After this time there is a nearly isotropic
and thermal Quark-Gluon Plasma.
