Malnutrition is associated with a delayed recovery from illness and an increased rate of complications. Heart failure, respiratory diseases, impaired immune function and postoperative wound healing are influenced by the nutritional status. Nutritional assessment includes patients history, physical examination, anthropometric measurements, laboratory dates and changes of immunocompetence. Anthropometric assessment, like skinfold and muscle area measurements, is not an extremely accurate method predicting nutritional status of an individual patient. Multifactorial influence on the concentration of the serum proteins, albumine, transferrin and retinolbinding protein caused a wide range of normal values. Therefore the validity and sensitivity of these parameters to assess nutritional status are diminished for the individual patient. The concentration of serum albumin seems to be of some value as a marker for nutritional status. The influence of malnutrition on immunity is complex. There are no sensitive markers available to assess the influence of malnutrition on the immuno competence of an individual patient for the clinical routine. Malnutrition may be assessed and the resulting clinical complications may be predicted by calculating a prognostic nutritional index, which include several nutritional parameters.