A review of enteral tube feeding formulations is presented. When choosing an enteral formula product for a patient one must first determine the calorie and protein needs of the patient. Then one must determine the level of gastrointestinal tract function. First ask, to what degree is the small bowel functioning: totally, partially, or not at all? Has the small bowel lost surface area because of atrophy, inflammation, or surgical removal? Is the gut edematous because of hypoalbuminemia or congestive heart failure? Is bowel motility impaired by opioids, anticholinergics, or mechanical ileus? Is digestion limited by pancreatic or bile acid insufficiency? Has absorption been decreased by intestinal ischemia or gastrointestinal bleeding? Decreased surface area, bowel edema, and diminished digestive juices are only partial losses of small bowel function. An elemental formula should be tried first in these situations. Any of the other problems alone or in combination probably preclude the use of the small bowel and the patient will need total parenteral nutrition. If gastrointestinal function is adequate, then other organ failures that result in specific nutrient intolerance must be ruled out. If gastrointestinal function is adequate and no other organ failures preclude the use of a polymeric formula, then one must decide if stress and hypermetabolism are present. Enteral feeding is the preferred method of providing specialized nutrition support. Bowel rest reduces the barrier functions of the gut and malnutrition reduces cell-mediated immunity. The indications and relative contraindications for enteral tube feeding are also reviewed. The rationale for the formula design and the evidence for formula efficacy are presented. Polymeric, elemental, organ-specific, and immune-modulating formulas are discussed. Guidelines for formula selection are suggested.