This chapter addresses the question, “How can health be measured?” To understand and assess purported answers, something needs to be said about what health is. From a theoretical perspective, what is central to health is the functional efficiency of parts and processes of the body and mind, not what one says on the basis of the functional efficiency of parts concerning overall health. However, for the practical purposes of measurement, what matters is the distress and activity limitations that health problems entail, not the nonevaluative determination of the extent to which parts and processes contribute to survival and reproduction. To define a measure of population health, health states must be categorized and values or “quality weights” must be assigned to those health states. Both of these tasks are problematic, with the greatest problems lying in the assignment of values to health states. The quality weights to be assigned to health states are determined by eliciting preferences among health states from a random sample of members of the population. But the precision, accuracy, and relevance of the resulting values are questionable.