The rate of cancer patients will continue to increase in the coming decades. Additionally, new treatments will continue to emerge. The increased survival rates and the anti-tumour medications used will without doubt contribute to an increased rate of patients with oncological emergencies. Patients with oncologic emergencies will foremost seek care at an emergency department, but physicians at other clinics should also be familiar with the initial management of these patients. The initial management of any critically ill and instable patient, including those presenting with an oncologic emergency, should be based on the O-ABCDE approach and aimed at countering established or potentially life-threatening conditions. Oncologic emergencies can be divided into several different categories. This chapter focuses on five different oncologic emergency categories: metabolic, haematological, cardiovascular, neurologic, as well as treatment related. While all critically ill patients with an oncologic emergency must be attended to based on the O-ABCDE approach; it is of vital importance for the treating physician to also treat the patients symptomatically and at the same time rule in or rule out other differential diagnosis.