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The truism that definitions matter has particular relevancy in the health sciences where definitions about diseases and other disorders guide both the identification of populations at risk and the development of remedial interventions. A good illustration of how the relationship works is provided by ProteinEnergy Malnutrition (PEM). Today, most investigators would agree that PEM includes a range of syndromes among infants and pre-school children who display growth and biochemical abnormalities produced by the synergistic effects of dietary deficiencies and various infections. The clinical syndromes are kwashiorkor and marasmus plus a range of mixed types usually designated as marasm ic kwashiorkor. Far more common, however, are sub-clinical cases that most often get bracketed together under the heading of mild-to-moderate PEM (Srikantia 1982; Torin and Viteri 1988). This definition of PEM has taken over 50 years of observation and research to achieve. It was preceded by a much simpler one that focused on kwashiorkor and its presumed cause of protein deficient diets. Using this relationship as their guide, investigators identified