The authors discuss some of the conceptual issues that must be considered in using and understanding psychiatric classification. DMS-IV is a practical and common sense nosology of psychiatric disorders that is intended to improve communication in clinical practice and in research studies. DSM-IV has no philosophic pretensions but does raise many philosophical questions. This paper describes the development of DSM-IV and the way in which it addresses a number of philosophic issues: nominalism vs. realism, epistemology in science, the mind/body dichotomy, the definition of mental disorders, and dimensional vs. categorical classification.