  Understanding the underlying mechanisms causing rapid thermalization deduced
for high-energy heavy ion collisions is still a challenge. To estimate the
thermalization time, entropy growth for classical Yang-Mills theories is
studied, based on the determination of Lyapunov exponents. Distinct regimes for
short, medium and long sampling times are characterized by different properties
of their spectrum of Lyapunov exponents. Clarifying the existence of these
regimes and their implications for gauge-field dynamics is one of the results
of this contribution. As a phenomenological application we conclude that for
pure gauge theories with random initial conditions thermalization occurs within
few fm/c, an estimate which can be reduced by the inclusion of fermions,
specific initial conditions, etc.
