All definitions of schizophrenia rely more or less on Kraepelin's hypothesis, Bleuler's theory or Schneider's pragmatic criteria. After a discussion on how these assumptions are referred to in classical and operational diagnostic systems in Europe, the results of a survey of 1983 literature are presented to show what kinds of assessments European workers currently employ. Lastly, guidelines are presented to enable psychopharmacological research to make the best use of the diagnostic systems to suit its purposes: essentially, which system or systems (the polydiagnostic approach for example) one should choose depends upon the symptoms the medication to be tested is supposed to treat, keeping in mind that patient sampling varies according to the system in question.