BACKGROUND & AIMS: Consensus regarding definitions of malnutrition and methods to assess nutritional state is lacking. We propose a definition and its operationalization. METHODS: A definition was formulated on the basis of the pathophysiology of malnutrition, while reviewing the metabolic and physiological characteristics of different populations, considered to be malnourished. The definition was operationalised to yield measures to perform nutritional assessment. RESULTS: Malnutrition was defined as "a subacute or chronic state of nutrition in which a combination of varying degrees of over- or undernutrition and inflammatory activity has led to a change in body composition and diminished function". Its operationalization led to four elements that may serve as the basis of nutritional assessment: (1) measurement of nutrient balance, (2) measurement of body composition, (3) measurement of inflammatory activity, and (4) measurement of muscle, immune and cognitive function. Most elements measured should be validated with gold standards; normal values should be obtained in different populations. Values obtained in people considered to be at nutritional risk should be related to outcome. CONCLUSION: A definition is proposed that reflects the pathophysiology of malnutrition and that, when operationalised, will lead to measures reflecting this pathophysiology. Such an approach may yield comparable and reproducible rates and degrees of malnutrition in populations as well as in individuals.