Time-dependent density functional theory relies on a one-to-one correspondence between external potentials in the N-particle Schrodinger equation and the one-particle densities they generate. Runge and Gross argued that this correspondence holds (up to a time-dependent constant in the potential) for sufficiently "nice" external potentials. I will discuss a mathematical setting for this argument and explain when it can be made fully rigorous. The interaction potentials play an important role for this and I will discuss the problems that arise in the presence of singular interactions, like the Coulomb potential. This is joint work with S. Fournais, M. Lewin, and T. Ostergaard Sorensen.