Malnutrition in children and adolescents may be underestimated during hospital stay. In western countries, children were often hospitalized for acute or chronic diseases that are not necessarily related to malnutrition. However, acute or chronic injuries may hamper nutritional status, prolonging recovery after admission and consequently length of hospital stay. Several methods and techniques are known to investigate malnutrition in children, even if their use is not widespread in clinical practice. Many of these are simple and easy to perform and could be useful to a better management of every kind of illness. In this review, we will focus on clinical tools necessary to reveal a nutritional risk at admission and to assess nutritional status in hospitalized children and adolescents.