  In 1959 Weibel demonstrated that when a QED plasma has a temperature
anisotropy there exist unstable transverse magnetic excitations which grow
exponentially fast. In this paper we will review how to determine the growth
rates for these unstable modes in the weak-coupling and ultrarelativistic
limits in which the collective behavior is describable in terms are so-called
"hard-loops". We will show that in this limit QCD is subject to instabilities
which are analogous to the Weibel instability in QED. The presence of such
instabilities dominates the early time evolution of a highly anisotropic
plasma; however, at longer times it is expected that these instabilities will
saturate (condense). I will discuss how the presence of non-linear interactions
between the gluons complicates the determination of the saturated state. In
order to discuss this I present the generalization of the Braaten-Pisarski
isotropic hard-thermal-loop effective action to a system with a temperature
anisotropy in the parton distribution functions. The resulting hard-loop
effective action can be used to determine the time and energy scales associated
with the possible saturation (condensation) of the gluonic modes. We will also
discuss the effects of anisotropies on observables, in particular on the heavy
quark energy loss.
