Although perioperative and intensive care has improved rapidly during the last decade, patients undergoing major surgery as well as patients in need of treatment in an intensive care unit are still at an especially high risk of developing life-threatening complications. These complications range from cardiocirculatory failure due to myocardial infarction or pulmonary embolism to sepsis with associated multiple organ failure. To manage these complications and provide the patient with fast and appropriate therapeutic interventions is the key task of any medical professional working in an acute and intensive care setting. This chapter offers an overview of the most common perioperative, post-anesthesia, and intensive care unit complications, as well as insights into their etiology, diagnosis, and treatment.