Nutritional management of the cancer patient cannot be ignored, given the well-established links between malnutrition and mortality, postoperative complications, toxicity of radiation and chemotherapy, and quality of life. The expertise of a medical specialist in nutrition is often required given the diversity of the clinical presentation of malnutrition in cancer patients and in certain areas lacking evidence in the literature. In at least some situations, such as for malnourished patients requiring surgery, preoperative nutrition (normally enteral nutrition if possible), is indicated and is also the subject of international recommendations/clinical practice guidelines. The role of the specialist in nutrition is to disseminate such information. There are some situations where the literature offers useful approaches, even if there are no guidelines/recommendations. For example, in disease for which there are not curative therapies but the patient is not yet in a terminal stage, it is legitimate to propose parenteral nutrition in the case of partial bowel obstruction, without waiting for advice from a nutrition specialist. By contrast, his advice is of interest in the ethical discussion which sometimes surrounds decisions to initiate artificial nutrition in patients in the terminal phase. Nutritional recommendations are difficult to formulate for patients with solid tumors on chemotherapy, because there are few data in the literature. The expertise of the specialist in nutrition in this case is most effectively deployed when active treatment of the disease is planned (earlier rather than later in the disease trajectory, in patients responding to treatment for whom further treatment is planned, participants in phase I studies). For cancers of the head and neck preoperative enteral nutrition is recommended for malnourished patients. The literature also provides some practical solutions, even in the absence of well-defined recommendations in some instances. Enteral nutrition is also indicated, without consulting a specialist in nutrition, during radiotherapy when intake falls, to prevent the development of malnutrition. Given the high frequency and important consequences of malnutrition in patients receiving combined radiation and chemotherapy, it is appropriate to initiate enteral nutrition at the 3rd week of treatment. The role of the specialist in nutrition is to interpret the literature and to develop procedures for specific cases.