The fundamental changes in body composition which accompany surgical illness are loss of body protein, gain in extracellular water, and erosion of body fat stores. The rate at which each of these processes occurs and the balance between them determine the clinical picture observed and an understanding of them opens the door to intelligent management of many complex disorders. Recent advances in technology have lent impetus to body composition research and a number of new findings have major implications for clinical practice. In this review, some of the work from the body composition laboratories in the University Department of Surgery at Auckland Hospital will be described with special emphasis on that which is directly relevant to the practising surgeon.