Based on the pioneering work of Emil Kraepelin, Karl Jaspers, Kurt Schneider and representatives of logical empiricism, the basic principles of the operational diagnostic approach in psychiatry are described. Operational diagnostic systems like ICD-10 and DSM-IV aimed at a standardisation of psychiatric language which can be accepted by different schools in psychiatry. However, ICD-10 and DSM-IV should not be misinterpreted as "nosology" because they do not reflect a specific model of psychiatric diseases. The advantages of operational diagnostic systems as instruments for communication in a clinical and scientific context are limited by the fact that they disregard the subjective psychopathology. This dimension, however, deserves attention in clinic and research.