Malnutrition is present in up to 80% of critically ill patients on admission or may develop during their stay in the ICU. Patients with prolonged ICU stays are at particularly high risk of developing secondary malnutrition associated with cumulative macro- and micronutrient deficiencies. Malnourished patients require careful consideration of the indications/ contraindications and individual metabolic tolerance of medical nutrition therapy (MNT). In particular, MNT also requires appropriate monitoring, which should include follow-up of nutritional status. However, to date, there are no structured recommendations for the (serial) assessment of the nutritional status of ICU patients. All specific tools/instruments/procedures have specific limitations (e.g. interferences by hydration state). Screening tools to determine nutritional status/risk on ICU admission include NUTRIC (Nutrition Risk in Critically Ill), NRS-2002 (Nutritional Risk Screening), MNA (Mini Nutritional Assessment), SGA (Subjective Global Assessment) and mGLIM (modified Global Leadership Initiative on Malnutrition). The GLIM definition of malnutrition, agreed by international professional societies in 2019, includes phenotypic and aetiological criteria that can be applied to critically ill patients. Laboratory and physical methods to determine nutritional status include anthropometry, creatinine/cystatin C ratio (CCR) and handgrip strength; phenotypic assessment may be based on body mass index (BMI), (serial) muscle sonography, computed tomography (CT) and bioelectrical impedance analysis (BIA). To assess nutritional status on admission to the ICU, the latest ESPEN Intensive Care Medicine guideline recommends a careful history, thorough examination, consideration of BMI and determination of body composition including muscle mass and strength.