  Quasiparticle dynamics in relativistic plasmas associated with hot,
weakly-coupled gauge theories (such as QCD at asymptotically high temperature
$T$) can be described by an effective kinetic theory, valid on sufficiently
large time and distance scales. The appropriate Boltzmann equations depend on
effective scattering rates for various types of collisions that can occur in
the plasma. The resulting effective kinetic theory may be used to evaluate
observables which are dominantly sensitive to the dynamics of typical
ultrarelativistic excitations. This includes transport coefficients
(viscosities and diffusion constants) and energy loss rates. We show how to
formulate effective Boltzmann equations which will be adequate to compute such
observables to leading order in the running coupling $g(T)$ of high-temperature
gauge theories [and all orders in $1/\log g(T)^{-1}$]. As previously proposed
in the literature, a leading-order treatment requires including both $2<->2$
particle scattering processes as well as effective ``$1<->2$'' collinear
splitting processes in the Boltzmann equations. The latter account for nearly
collinear bremsstrahlung and pair production/annihilation processes which take
place in the presence of fluctuations in the background gauge field. Our
effective kinetic theory is applicable not only to near-equilibrium systems
(relevant for the calculation of transport coefficients), but also to highly
non-equilibrium situations, provided some simple conditions on distribution
functions are satisfied.
