More than 10 million new cancer cases are detected each year worldwide, 95% of which are caused by predisposing factors, and of those, more than one third are linked to dietary factors as the main cause. The ability of maintaining an adequate nutritional status in oncohematologic patients is a common problem since the disease itself and the therapy may lead to a protein-caloric hyponutrition state that influence their quality of life and survival. For that reason, in this section we will focus on the prevalence and etiology of hyponutrition in oncologic patients, assessing the possible causes related with the tumor itself, with the patient and with administered therapies. We will also discuss performing a correct nutritional assessment in this type of patients and thus determining the main effects derived from hyponutrition status; finally, we will discuss the objectives of nutritional support and the best nutritional plan that will have to be adjusted to each patient.