The gut is an important target organ for stress caused by severe insults such as sepsis, trauma, burn, shock, bleeding and infection. Severe insult to the gut is considered to have an important role in promoting infectious complications and multiple organ dysfunction syndrome. These are sequelae of interactions between deteriorated intestinal epithelium, the immune system and commensal bacteria. The gut is the "motor" of multiple organ failure, and now it is recognized that gut dysfunction is a causative factor in disease progression. The gut flora and environment are significantly altered in critically ill patients, and the number of obligate anaerobes is associated with prognosis. Synbiotic therapy is a combination of probiotics and prebiotics. Probiotic, prebiotic and synbiotic treatment has been shown to be a promising therapy to maintain and repair the gut microbiota and gut environment. In the critically ill, such as major abdominal surgery, trauma and ICU patients, synbiotic therapy has been shown to significantly reduce septic complications. Further basic and clinical research would clarify the underlying mechanisms of the therapeutic effect of probiotic/synbiotic treatment and define the appropriate conditions for use.