This chapter discusses postoperative infection. To understand the postoperative infection, one must have the basic knowledge of how infection occurs, and the mechanism of postoperative infection. Classification of postoperative infection is also discussed in this chapter. Postoperative bacterial growth occurs in hematoma and seroma which are the culture media for bacteria, therefore meticulous hemostasis is important. Newer developments are to prevent bacterial adherence which is the main cause of infection. Chronic osteomyelitis is classified and treated as for hematogenous chronic osteomyelitis. Acute post-traumatic osteomyelitis is bone infection that result in traumatic injury, with proliferation of bacteria and expression of infection.