  We propose a method to study the transition to chaos in isolated quantum
systems of interacting particles. It is based on the concept of delocalization
of eigenstates in the energy shell, controlled by the Gaussian form of the
strength function. We show that although the fluctuations of energy levels in
integrable and non-integrable systems are principally different, global
properties of the eigenstates may be quite similar, provided the interaction
between particles exceeds some critical value. In this case the quench dynamics
can be described analytically, demonstrating the universal statistical
relaxation of the systems irrespectively of whether they are integrable or not.
