This chapter examines the assessment and measurement of mental disorders. It explores the special problems that have arisen in epidemiologists' attempts to transfer diagnostic criteria from the domain of clinical evaluation to the much different epidemiological arena in which disorder is measured in the general population by survey. Some early psychiatric epidemiological studies were based on the assumption that clinical prevalence could be used to validly infer the community population's true prevalence. The most notable attempt in Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) to make general progress on the false positives problem has been the development of a clinical significance (CS) criterion for use in evaluating mental disorder. The chapter also presents an example of a recent attempt to use DSM derived criteria in an epidemiological study to measure the prevalence of a particular mental disorder, namely, major depressive disorder (MDD).