Pediatric oncologic patients are treated according to a variety of treatment regimens, depending on the type of malignancy. The complications associated with treatment can be divided into those related to change in immune status/infection-related complications and complications directly related to the toxicity of the treatment. Acute toxicities include myelosuppression, alopecia, nausea and emesis, mucositis, liver function disturbances, and allergic reactions. Infections are related to immunosuppressive therapy and to the disruption of the mucocutaneous integrity by local tumor invasion, insertion of foreign bodies such as vascular catheters, surgery, radiotherapy, and cytotoxic chemotherapy. In this chapter the complications for which imaging can play a role will be discussed in general and separately per anatomic system or location