Malnutrition in its various forms is without doubt the commonest disease. Acute malnutrition resulting from a total lack of ingestion clearly shows that not only are water and electrolytes a priority for survival but also energy reserves. It also shows that the maximum wasting that the lean body mass can withstand is around 40% before the organism collapses and death occurs. Malnutrition is more often the results of insufficient ingestion, and the term protein-calorie malnutrition has been suggested for the most frequently occurring type. This can give rise to confusion because the deficiency in these patients is not limited to these macronutrients but extends, rather, to micronutrients, which are equally important to the functioning of the body. A useful approach for the surgeon is to analyze how the various organs and systems have been affected. The effects on the gastrointestinal tract, the immunologic system, and formation of scar tissue are reviewed.