Until relatively recently, the gastrointestinal (GI) tract was considered a dormant, metabolically and immunologically inactive organ in critically illnesses. However, the GI tract provides a number of crucial functions that, in fact, may influence morbidity and mortality of many critically ill patients. Its large absorptive area provides a site for nutrient digestion and utilization and serves as an important barrier preventing the systemic absorption of intraluminal microbes and its toxic products. Moreover, the GI tract is the largest reservoir of lymphocytes in the body, which significantly contribute to the immune response of the critically ill patients. The gut dysfunction occurs frequently and early in the intensive care patients. Abnormal colonization, impaired intestinal epithelial barrier function and bacterial translocation represent the key components of gut failure implicating in the pathogenesis of sepsis and multiorgan dysfunction. This review summarizes recent insights into the role of the gut in critically ill patients with particular focus on 1) the basis of "gut-origin hypothesis", 2) pathophysiology of gut dysfunction, 3) monitoring of intestinal function, and 4) protective measures and novel therapeutic strategies.