Introduction; Classification and research: Historical and theoretical aspects; 1. Issues in classification: Part I. Some Historical Aspects: 2. Issues in classification: Part II. Classifying Melancholia: 3. Issues in classification; Part III. Utilizing Behavioral Constructs in Melancholia Research: 4. Issues in classification: Part IV. Some Statistical Aspects: Development and Validation of a Measure of Psychomotor Retardation as a Marker of Melancholia: 5. Psychomotor change as a feature of depressive disorders: historical overview and current assessment strategies; 6. Development and structure of the CORE system; 7. Reliability of the CORE measure; 8. Validity of the CORE: Part I. A Neuroendocrinological Strategy: 9. Validity of the CORE: Part II. Neuropsychological Tests: 10. Validity of the CORE; Part III. Outcome and Treatment Prediction: 11. Phenotypic expression of melancholia contrasted for those with bipolar and unipolar illness courses; 12. Psychotic depression: clinical definition, status and the relevance of psychomotor disturbance to its definition; 13. A clinical algorithm for defining melancholia: comparison with other sub-typing measures; 14. Rating the CORE: a user's guide; The Neurobiology of Melancholia: 15. Melancholia as a neurological disorder; 16. Melancholia and the aging brain; 17. Magnetic resonance imaging in primary and secondary depression; 18. Functional neuroimaging in affective disorders; 19. Summary and conclusions; Appendix; The CORE measure: procedural recommendations and rating guidelines; Bibliography.