Health status is hard to measure. It is widely recognized that health is multi-dimensional reflecting the combination of an array of factors that include physical, mental and social well-being, genotype and phenotype influences as well as expectations and information. A multitude of health indicators have been used in scientific studies drawing on data from both the developed and developing world. Understanding what those indicators measure is central if the results reported in the studies are to be interpreted in a meaningful way. Whether one is interested in summarizing the health of a population or understanding the links between health and other measures of well-being at the individual level, poor measurement will likely yield poor inferences. There is a large literature that discusses the validity and limitations of different health measures. Murray and Chen (1992) and Sadana (2001) provide excellent reviews and discussion. Some of the most insightful empirical studies have compared indicators of specific morbidities reported by respondents in health interview surveys with indicators based on health examinations of the same individuals conducted by trained health workers. Other very influential studies have examined the extent to which self-reported health predicts health problems later in life. A third class of studies contrasts prevalence rates based on health interviews with other sources.