Fracture healing is a highly complex regenerative process that is essentially a replay of developmental events. These events include the action of many different cell types, a myriad of proteins, and active gene expression that in the majority of cases ultimately will restore the bone's natural integrity. Several biologic and biophysical approaches have been introduced to minimize delayed healing and nonunions, some with promising results. One example of such an approach is low intensity pulsed ultrasound, a noninvasive form of mechanical energy transmitted transcutaneously as high frequency acoustical pressure waves in biologic organisms. Numerous in vivo animal studies and perspective double blind placebo controlled clinical trials have shown that low intensity ultrasound is capable of accelerating and augmenting the healing of fresh fractures. Preliminary evidence suggests efficacy in the treatment of delayed healing and nonunions as well. This article reviews the animal and clinical studies that consider the effects of ultrasound on fracture healing, and the in vivo and in vitro work that strives to identify the biologic mechanism(s) responsible for the ultrasound induced enhancement of osteogenesis and fracture healing.