Chapter 10, more than any other, makes this textbook unique. It explains the physics underlying quantum phenomena, with the aim of giving the student a clearer picture of quantum behavior and its causes, and thus making it easier for them to understand the further reading. The phenomenological approach of stochastic quantum mechanics, inspired by Nelson’s theory, shows the Markovian nature of the quantum description. The discussion then points to the vacuum or zero-point field mentioned in Chapter 2, as the source of the quantum stochasticity and characteristic wave-like phenomena. The theory thus developed, stochastic electrodynamics, is shown to provide a first-principles explanation for the operator formalism and related quantum postulates. The chapter concludes with a brief introduction of the hydrodynamic quantum analogue of a walking droplet, a striking macroscopic example of a particle-wave system capable of expressing quantum-like features under appropriate conditions.