This chapter discusses biology of distraction osteogenesis. Distraction Osteogenesis is the production of new bone between vascular bone surfaces which are separated through gradual distraction. When fracture callus is slowly distracted, new bone is formed in the gap, this procedure is called distraction osteogenesis, hitherto unknown phenomenon to the biologists, discovered by GA Ilizarov. Using modular ring external fixators and transosseous wires tensioned to stabilize the bone fragments. Distraction osteogenesis has been demonstrated to regenerate living bone, capable of bearing load, at a rate of 1 cm per month in children and 1 cm per 2 months in adults. Types of distraction osteogenesis and complications of distraction osteogenesis are also discussed in this chapter.