The time evolution following a quantum quench, i.e. an instantaneous change in the parameters of an isolated quantum system, has been shown to lead, in a wide range of models and under quite general conditions, to stationary behaviour with some thermal characteristics.  After a review of free field theory results that introduces necessary new concepts and tools, we investigate a quantum quench in the phi^4 model in a self-consistent approximation.  A direct application of perturbation theory is not suitable in this case and resummation of Feynman diagrams is needed.  We propose an ansatz for the long time behaviour of the system, which we verify by a self-consistency calculation and numerical integration of the equations of motion.