  The time of arrival at an arbitrary position in configuration space can be
given as a function of the phase space variables for the Liouville integrable
systems of classical mechanics, but only for them. We review the Jacobi-Lie
transformation that explicitly implements this function of positions and
momenta. We then discuss the recently developed quantum formalism for the time
of arrival. We first analyze the case of free particles in one and three space
dimensions. Then, we apply the quantum version of the Jacobi-Lie transformation
to work out the time of arrival operator in the presence of interactions. We
discuss the formalism and its interpretation. We finish by disclosing the
presence (absence) of "instantaneous" tunneling for thin (thick) barriers.
