A new approach to the dynamics of relaxation and kinetics of thermaliz
ation in a scalar held theory is presented that incorporates the relev
ant time scales through the resummation of hard thermal loops. An alte
rnative derivation of the kinetic equations for the ''quasiparticle''
distribution functions is obtained that allows a clear understanding o
f 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-shel
l effects, and contributions that change chemical equilibrium on short
time scales. As a by-product of these methods we establish the equiva
lence 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 approxima
tion'' including hard thermal loop effects. Hard thermal loop resummat
ion dramatically modifies the scattering rate for long wavelength mode
s as compared to the usual (semi)classical estimate. Relaxation and ki
netics are studied both in the unbroken and broken symmetry phases of
the theory. The broken symmetry phase also provides the setting to obt
ain the contribution to the kinetic equations from processes that invo
lve decay of a heavy scalar into light scalar particles in the medium.