Magnetic resonance imaging (MRI) can be used to assess metabolically active components of FFM. MRI-derived organ mass (OM) explains part of the interindividual variance in REE. It may also add to our understanding of malnutrition- and obesity-related variance in REE. There is need for (i) standardisation of MRI technology in body composition research, (ii) reference data on detailed body composition including also more recent autopsy data, (iii) reducing the number of assumptions in model-based predictions and (iv) a combination of imaging technologies with in vivo measurements of individual OM respiration (e.g. by positron emission tomography).