Loss of appetite is part of the so-called "sickness behavior", which is a uniform reaction to many diseases, mainly triggered by inflammatory signals. Whereas this pattern may have been an evolutionary advantage long time ago, loss of appetite with consecutive malnutrition nowadays represents more a collateral damage of many acute and chronic diseases.Protein-energy-malnutrition should be diagnosed with the recently globally consented criteria of the "Global Leadership Initiative on Malnutrition" (GLIM). These criteria comprise 3 phenotypic and 3 etiologic criteria. If one criterium of each group is fulfilled, the diagnosis can be made.Besides the disease related loss of appetite, there are many potential causes of malnutrition. The entire spectrum may reach from malignancies to insufficient support in functionally impaired older subjects. Frequently, several factors may play a role. The very complex spectrum of potential and frequent causes of malnutrition has been summarized in the DoMAP model.Malnutrition may lead to severe consequences such as muscle atrophy and sarcopenia, leading to falls and fractures, impaired function of the immune system with an increase of infections and impaired wound healing, all of which are connected with increased morbidity and mortality.Many meta-analyses and guidelines have demonstrated the effectiveness of nutritional therapy in malnourished patients. There is good evidence for various approaches, that may be utilized alone or in combination, depending on the situation and disease of each individual patient. Particularly, the EFFORT-study, a large prospective randomized controlled trial, has demonstrated, that a structured, but individualized approach may be most effective.