This chapter covers the classification of common complications in surgery. It is specific to the pediatric population and highlights not only the presentation and treatment of these complications but also their prevention starting at the preoperative stage. We begin by discussing the common complication of infection with reference to the UK National Institute for Clinical Excellence guidance. Bleeding is discussed extensively and the chapter then progresses to general early and late postoperative complications. We then discuss procedure and site-specific complications with common tumors including neuroblastoma and Wilm’s tumor, thoracic surgery, and central venous catheters.