  A new approach to the dynamics of relaxation and kinetics of thermalization
in a scalar field theory is presented that incorporates the relevant time
scales through the resummation of hard thermal loops. An alternative derivation
of the kinetic equations for the ``quasiparticle'' distribution functions is
obtained that allows a clear understanding of the different ``coarse graining''
approximations usually involved in a kinetic description. This method leads to
a systematic perturbative expansion to obtain the kinetic equations including
hard-thermal loop resummation and to an improvement including renormalization,
off-shell effects and contributions that change chemical equilibrium on short
time scales. As a byproduct of these methods we establish the relation between
the relaxation time scale in the linearized equation of motion of the
quasiparticles and the thermalization time scale of the quasiparticle
distribution function in the ``relaxation time approximation''. Hard thermal
loop resummation dramatically modifies the scattering rate for long wavelength
modes as compared to the usual (semi) classical estimate. Relaxation and
kinetics are studied both in the unbroken and broken symmetry phases of the
theory. The broken symmetry phase also provides the setting to obtain the
contribution to the kinetic equations from processes that involve decay of a
heavy scalar into light scalar particles in the medium.
