Due to recent progress with cold atomic gases, the issue of how   a many-particle system prepared in a non-equilibrium initial state evolves in time, and how it thermalizes if  at all, has become an active topic of research. In this talk, by means of a simple model, namely an isolated quantum system described by a scalar phi-4 theory in d spatial dimensions, I will highlight some interesting theoretical issues. One of them is the role of the quantum critical point of the system which I will show strongly affects even the (macroscopic) short time dynamics by resulting in a quantum ageing behavior with universal exponents. This short time behavior is eventually dynamically destabilized by self-generated dissipation leading to eventual thermalization.