Nutritional deficiency is a common medical problem that affects 15-40% of cancer patients. It negatively impacts their quality of life and can compromise treatment completion. Oncological therapies, such as surgery, radiation therapy, and drug therapies are improving survival rates. However, all these treatments can play a role in the development of malnutrition and/or metabolic alterations in cancer patients, induced by the tumor or by its treatment. Nutritional assessment of cancer patients is necessary at the time of diagnosis and throughout treatment, so as to detect nutritional deficiencies. The Patient-Generated Subjective Global Assessment method is the most widely used tool that also evaluates nutritional requirements. In this guideline, we will review the indications of nutritional interventions as well as artificial nutrition in general and according to the type of treatment (radiotherapy, surgery, or systemic therapy), or palliative care. Likewise, pharmacological agents and pharmaconutrients will be reviewed in addition to the role of regular physical activity.