SummaryProblems with fracture healing are not uncommon. Bone stimulators apply an electromagnetic or ultrasonic signal to the bone to promote healing. Both in vitro and in vivo studies demonstrate effects on bone cell physiology and fracture healing. Direct current stimulates bone formation in tissue and organ culture experiments and in animal studies in a dose-response fashion. Electromagnetic fields alter the effects of hormones on the cell membranes, and increase production of growth factors and receptors, affect calcium flux across membranes, and stimulate endothelial cell proliferation and capillary formation. Animal studies show that pulsed electromagnetic fields speed bone repair and prevent disuse osteopenia. Low intensity ultrasound promotes cartilage calcification and alters gene expression. Clinical series suggest that electromagnetic stimulation is equivalent to bone grafting for initial treatment of long bone nonunion, and better in cases of failed surgery. Prospective, randomized, double blind studies show that noninvasive electrical stimulation is an effective treatment for long bone nonunion, and that ultrasonic treatment speeds the healing of fresh fractures.