Recently, it has been realized that in both two and three dimensions, time-reversal invariant band insulators come in two distinct phases: ordinary band insulators and topological band insulators. These can be distinguished by the presence or absence of protected gapless modes at their boundaries. This talk will address the question of whether an analogous distinction exists for strongly interacting time-reversal invariant insulators. More specifically, is there a "fractional" analogue of the 3D topological band insulator, in the same way that there is a fractional analogue of the integer quantum Hall effect? To show that the answer is yes, I will describe a family of model Hamiltonians whose low-energy excitations are fractionally charged fermions, and which have gapless fractionally charged fermionic surface excitations.