Hydrated lime (HL) is an active filler in some bitumens. Rheological models demonstrate that HL interacts with certain bitumens to develop an adsorbed (interactive) layer around the HL particles. The volume of this layer can be substantial, causing HL to have a much more substantial effect on the bitumen than inert fillers. The level of interaction between HL and bitumen is bitumen dependent. This interaction causes HL to strongly affect high-temperature rheology in certain bitumens, but it has less of an effect in others. The low-temperature stiffening effects of HL are less prominent, and a significant level of fracture toughening (at low temperatures) occurs through the addition of HL. The active nature of HL with certain bitumens is further verified by nuclear magnetic resonance. HL is considered a multifunctional additive with potential benefits to the binder and to the mixture that include resistance to deformation, low-temperature fracture toughening, and increased resistance to age hardening. HL may be competitive with some polymer additives.