Traditionally, evaluation of intestinal function has been limited largely to monitoring gastric pH and intestinal motility. This clinical approach has led clinicians to equate normal intestinal motility with normal intestinal function and to assume that if stress-induced gastric bleeding can be prevented, all will be well. However, it is becoming increasingly clear that the gastrointestinal tract is not a passive organ and that intestinal dysfunction is not limited to ileus and upper gastrointestinal bleeding. Instead, the gastrointestinal tract is recognized as having important endocrine, metabolic, immunologic, and barrier functions, as well as its traditional role in nutrient absorption. Over the last 5 years, there has been a resurgence of interest in the role of intestinal barrier failure in the development of systemic infection and multiple organ failure in the critically ill or injured patient.