  We present kinetic equations that describe the evolution of O(N)-symmetric
real scalar quantum fields out of thermal equilibrium in a systematic
nonperturbative approximation scheme. This description starts from the
1/N-expansion of the 2PI effective action to next-to-leading order, which
includes scattering and memory effects. From this starting point one is lead to
evolution equations for the propagator, which are nonlocal in time. Numerical
solutions showed that the propagator depends only very slightly on the center
coordinates already after moderate times, and that correlations between earlier
and later times are suppressed exponentially, which causes an effective memory
loss. Exploiting these two observations, we combine a first order gradient
expansion with a Wigner transformation to derive our kinetic equations, which
are local in time, from the nonlocal evolution equations. In contrast to
standard descriptions based on loop expansions, our kinetic equations remain
valid even for nonperturbatively large fluctuations. Additionally, employing a
quasi-particle approximation, we eventually arrive at a generalized Boltzmann
equation.
