A Completion of Quantum Mechanics

A proposal of how to complete non-relativistic quantum mechanics to a physically meaningful, mathematically precise and logically coherent theory is reviewed. Our proposal leads to a general, non-linear stochastic law for the time-evolution of states of individual physical systems. An application of the general formalism to the quantum theory of fluorescence of an atom coupled to the radiation field is sketched. Some remarks on relativistic quantum theory conclude our review.