In the industrialised countries, the sedentary life and the ageing process of the population, as well as the more frequent chronic diseases and heavy treatments, increase the incidence of protein-energy malnutrition (PEM). The insidious nature and harmful outcome of the PEM on the recovery process requires careful attention of the practitioner to the clinical signs of PEM. Their detection includes an anamnesis, anthropometric examinations, and assessments of the nutritional intakes and the impact of disease and medico-surgical treatments. However, the loss of muscle mass, which is the main indicator of the PEM, is often only assessed by the measurement of the body composition using bioelectrical impedance analysis. The advantage of this method is to distinguish fat-free mass, including muscle mass, from fat mass, when the loss of muscle is hidden by an increase of fat mass and/or body water. Using these different tools allows the practitioner to early detect PEM, to identify its causes, and to establish an appropriate nutritional schedule, in order to prevent from PEM or correct it.