One of the challenges during chemotherapy and radiotherapy is to complete the planned cycles and doses without dose-limiting toxicity. Growing evidence clearly demonstrates the relationship between dose-limiting toxicity and low muscle mass. Moreover, malnutrition leads to low performance status, impaired quality of life, unplanned hospital admissions, and reduced survival. In the past, the lack of clear and authoritative recommendations and guidelines has meant that oncologists have not always fully appreciated the importance of nutritional therapy in patients receiving anticancer treatments. Therefore, collaboration between oncologists and clinical nutrition specialists needs to be urgently improved. Recent guidelines from scientific societies and practical recommendations by inter-society consensus documents can be summarized as follows: 1) timely nutritional therapy should be carefully considered if patients undergoing anticancer treatments are malnourished or at risk of malnutrition due to inadequate oral intake; 2) if oral intake is inadequate despite counseling and oral nutritional supplements, supplemental enteral nutrition or, if this is not sufficient or feasible, parenteral nutrition should be considered; 3) home artificial nutrition should be prescribed and regularly monitored using defined protocols developed between oncologists and clinical nutrition specialists; 4) appropriate nutritional management in the context of simultaneous care should become a guaranteed right for all patients with cancer. The purpose of this review is to provide oncologists with an overview of the aims and current evidence about nutrition in oncology, together with updated practical and concise recommendations on the application of nutritional therapy in cancer patients receiving chemoradiotherapy.