In this chapter we shall come back to the issue of the undulatory nature of matter. Interferometer experiments with a blocked path show that this character cannot be taken in a classical sense. Indeed, quantum probabilities are deeply affected by the presence of features. The possibility of measurements without interaction shows that such probabilities have in quantum mechanics an ontological import. We then state a complementarity between local events and non-local features. Delayed choice experiments sets these aspects in new and enlightening terms.