What do we mean when we say that a mental condition is a medical disorder rather than a normal form of human suffering or a problem in living? The status of psychiatry as a medical discipline depends on a persuasive answer to this question. The answers tend to range from value accounts that see disorder as a sociopolitical concept, used for social control purposes, to scientific accounts that see the concept as strictly factual. I have proposed a hybrid account, the harmful dysfunction (HD) analysis, that incorporates both value and scientific components as essential elements of the medical concept of disorder, applying to both physical and mental conditions. According to the HD analysis, a condition is a disorder if it is negatively valued ("harmful") and it is in fact due to a failure of some internal mechanism to perform a function for which it was biologically designed (i.e., naturally selected). The implications of this analysis for the validity of symptom-based diagnostic criteria and for challenges in cross-cultural use of diagnostic criteria are explored, using a comparison of the application of DSM diagnostic criteria in the U.S. and Taiwan.