Abstract: Certain spectral invariants of a Dirac operator provide geometric refinements of its index. These include the eta-invariant and determinant line, which are, respectively, the degree 1 and 2 refinements (so to speak). I'll review these two invariants and then consider the so-called index gerbe, which is the degree 3 refinement. After reviewing the geometry of gerbes, I'll discuss a particular model for the index gerbe and what the model tells us about the geometry of this gerbe.