Dynamical tunneling is a phenomenon that can be observed in certain dynamical systems at the borderline of quantum and classical dynamics. Rather than tunneling through an energy barrier (as in the case of “conventional” tunneling), a different constant of motion forbids this classically forbidden process. It is mostly observed in dynamical systems that exhibit a mixed classical phase space. In a mixed phase space, the dynamics of a particle can be either chaotic or regular depending on the initial position and momentum of the particle.