Patients in intensive care units (ICUs) are subject to many complications connected with the advanced therapy required for their serious illnesses. Complications of ventilatory support include problems associated with short-term and long-term intubation, barotrauma, gastrointestinal tract bleeding, and weaning errors. Cardiac tachyarrhythmias can arise from a patient's intrinsic cardiac disease, as well as from drug therapy itself. Hemodynamic monitoring is crucial to careful patient management, but it is associated with technical complications during insertion such as pneumothorax, as well as interpretive errors such as those caused by positive end-inspiratory pressure. Acute renal failure can develop as a result both of therapy with drugs such as aminoglycosides and hypotension of many etiologies, as well as the use of contrast media. Nosocomial infection, which is a dreaded complication in ICU patients, usually arises from sources in the urinary tract, bloodstream, or lung. Complications frequently can arise if the interactions of drugs commonly used in the ICU are not recognized. Further, the ICU patient is subject to nutritional complications, acid base problems, and psychological disturbances. This monograph deals with the frequency, etiology, and prevention of these common ICU complications.