  Hydrodynamical analysis of experimental data of ultrarelativistic heavy ion
collisions seems to indicate that the hot QCD matter created in the collisions
thermalizes very quickly. Theoretically, we have no idea why this should be
true. In this proceeding, I will describe how the thermalization takes place in
the most theoretically clean limit -- that of large nuclei at asymptotically
high energy per nucleon, where the system is described by weak-coupling QCD. In
this limit, plasma instabilities dominate the dynamics from immediately after
the collision until well after the plasma becomes nearly in equilibrium at time
t \alpha^(-5/2)Q^(-1).
