In this presentation, I'll review the concepts associated with the phenomenon of the eigenstate thermalization---the effective indistinguishability of the eigenstates of a quantum ergodic system.

The eigenstate thermalization replaces the defocussing of classical trajectories as an underlying mechanizm for the loss of memory of the initial conditions. I will further illustrate the geometric structures behind the phenomenon and indicate the possible scenarios for how the quantum ergodicity can disappear.