The low-temperature fracture toughness was determined for rubber-modified asphalt binders. Crumb rubber tire, both plain and surface modified, and devulcanized rubber tire were investigated for their effectiveness in improving low-temperature asphalt binder performance in a notched, three-point bending beam fracture test. The increases in low-temperature fracture toughness for unmodified crumb rubber-asphalt mixtures were highest for fine ground rubber. At loading levels of 4 to 10 wt percent on the binder, the fracture toughness for 10-and 20-mesh ground rubber tire-modified samples was not significantly different from that of the unmodified binders. Reasonable increases in toughness were found for binder modification with 4 to 10 wt percent 30-, 40-, and 80-mesh rubber samples. An in situ sulfur grafting reaction of low-molecular-weight polybutadiene onto the crumb rubber dramatically increases the fracture performance for both coarse and fine crumb rubber-modified binders. A 9-and 15-fold increase in added fracture toughness was observed for binders containing 7 wt percent of 10-and 20-mesh cryogenically ground rubber tire. Whether these improvements will translate into a reduction in transverse thermal stress cracking remains to be investigated in further tests on binder-aggregate mixes and in field trials. The fracture toughness for a devulcanized rubber-modified asphalt was also investigated. The homogeneous sample contained as much as 10 wt percent devulcanized rubber tire but performed only marginally better than an 85-100 penetration control sample. Reaction of the devulcanized rubber-modified binder with sulfur increases its fracture toughness by 18 percent.