  In classical statistical mechanics there is a clear correlation between
relaxation to equilibrium and chaos. In contrast, for isolated quantum systems
this relation is -- to say the least -- fuzzy. In this work we try to unveil
the intricate relation between the relaxation process and the transition from
integrability to chaos. We study the approach to equilibrium in two different
many body quantum systems that can be parametrically tuned from regular to
chaotic. We show that a universal relation between relaxation and
delocalization of the initial state in the perturbed basis can be established
regardless of the chaotic nature of system.
