This chapter discusses on nonunion of fractures of long bones. Nonunion of a fracture of long bone is a state in which all the healing processes have come to a halt as judged by clinical and roentgenographic evidence, beyond the stipulated period of healing for a particular bone due to mechanical or biologic failure, with a gap being filled with the fibrous or dense fibrocartilaginous tissue, usually requiring a change in treatment. Open fractures with infection perhaps are the most common causes of nonunion. The second common cause is an infection after open reduction and internal fixation (ORIF). Causes of nonunion may be divided into two types i.e. a mechanical failure and a biological failure. AO classification and Paley’s modification of Ilizarov’s classification are the two classifications of aseptic nonunion which are briefly described in this chapter. In recent years, many advances have been made in the treatment of nonunion with or without infection. Healing of fracture, correcting the deformity, mobilization of the adjacent stiff joints and complete eradication of infection are the objective of nonunion therapy. A detailed discussion on management of nonunion of fractures by Ilizarov method has been presented in this chapter. Infected nonunion (INU) is one of the greatest problems in orthopedics. Its incidence is more in India because of the high incidence of road traffic accidents. Plating, nonunion of the fractures of the tibia, patella, femur, pelvis and acetabulum, proximal humerus, shaft of humerus, and hip joint are the other topics which are also briefly covered in this chapter.