In the era of operational psychiatry, where mental disorders are defined on sets of polythetic diagnostic criteria and diagnoses regularly are derived from algorithms of structured diagnostic interviews, diagnostic assessment is often reduced to a matter of ‘symptom counting’, i.e., only checking symptoms listed as criteria in the diagnostic manuals. This has led to a gradual disappearance of the basic clinical ability of making a comprehensive, semi-structured diagnostic interview. The purpose of our chapter is to reintroduce the clinical and phenomenologically oriented, semi-structured interview for psychopathological and differential-diagnostic assessment, which we consider the only adequate method for such an assessment. Here, we present the ingredients of this interview, describe how it ideally should be conducted, what information it must gather, and how all gathered information finally must be synthesized to make a comprehensive differential-diagnostic evaluation. We also explicate important epistemological, psychopathological, and phenomenological issues that are at stake in the interview (e.g., the notions of psychopathological Gestalt, prototype, epoché, and the psychopathological eidetic reduction) as well as highlight certain pitfalls the clinician must be attentive to when conducting the clinical diagnostic interview.