Postoperative infections are classified into surgical site infections and remote infections. The risk factors associated with postoperative infections are related to medical practice, the patient, and bacteria. Postoperative infections not only prolong hospitalizations and increase patient burden but also affect prognosis. It is therefore important to conduct surveillance of postoperative complications at each facility to elucidate the risk factors for infection and to implement and evaluate infection control measures. Moreover, with the aging of the population, more surgeries are being performed in elderly individuals. However, the elderly tend to have age-related declines in the function of major organs as well as comorbidities, and surgeries in them carry a high risk despite advances in operative techniques and perioperative management. It is therefore important to determine surgical indications carefully by sufficiently assessing the risk preoperatively and performing appropriate perioperative management.