Cancer and its treatment result in severe biochemical and physiological alterations associated with a deterioration of quality of life (QoL). Cancer-related malnutrition may evolve into cancer cachexia due to complex interactions between pro-inflammatory cytokines and the host metabolism. Depending on the type of cancer treatment (either curative or palliative), the clinical condition of the patient and nutritional status, adequate and patient-tailored nutritional intervention should be prescribed (diet counseling, oral supplementation, enteral or total parenteral nutrition). Nutritional support has been widely advocated as adjunctive therapy for a variety of underlying illnesses, including surgery and medical oncotherapy (radiation or chemotherapy for cancer). Glutamine, n-3 fatty acids and probiotics/prebiotics are therapeutic factors that potentially modulate gastrointestinal toxicity related to cancer treatments. Enteral and parenteral nutrition may help improve patient survival, functional status and QoL, yet the benefits appear to be primarily limited to patients with good functional status and with gastrointestinal disease affecting nutritional intake. Parenteral nutrition offers the possibility of increased or maintenance of the nutrient intake in patients for whom normal food intake is inadequate and for whom enteral nutrition is not feasible, is contraindicated or is not accepted by the patient. This article reviews evidence on issues relevant to enteral and parenteral nutrition in patients with cancer.