Many factors can modify nutritional status in cancer patients, including cachexia, nausea and vomiting, decreased caloric intake or oncologic treatments capable of determining malabsorption. Cachexia is a complex disease characterized not only by a poor intake of nutrients or starvation, but also by metabolic derangement. Nausea and vomiting may limit the nutrient intake and are most often the consequences of oncologic treatments or opioid chronic therapy. Decreased caloric intake is considered to be one of the major causes of malnutrition, although the causes of anorexia remain unclear. Malabsorption is generally attributed to the consequences of oncologic treatments reducing the gastrointestinal absorption. Biochemical measurements and immunological tests may be not reliable indicators of nutritional status in cancer patients. Therefore, medical history, physical examination, estimates of daily oral intake, weight changes and an appropriate consideration of the nutritional requirements according to the stage of disease must still be assessed. The therapeutic approaches should be individualized and realistic. Whenever possible, oral nutrition is the method of choice, with due consideration for specific dietary needs. Nausea and anorexia can be reduced by different kinds of drugs. A careful decision based on good clinical judgement is necessary before deciding to start either enteral or parenteral nutrition, to avoid a useless, costly and difficult treatment. In choosing the route for administration of nutrients, availability of and access to a functioning gastrointestinal tract, compliance and comfort of the patient, gastrointestinal toxicity due to chemotherapy or radiotherapy fields, different costs, duration and place of treatment should be considered rather than the different capacity of parenteral versus enteral nutrition. However, postoperative periods after massive intestinal resection often require prolonged parenteral nutrition. The benefits of parenteral nutrition are not often demonstrable in patients with bowel obstruction. Different ethical aspects are presented. Flexibility in attempting to meet the nutrition needs of each patient is probably the most useful guide.