No counselling theory and practice can manage without diagnoses. Otherwise the counsellor would have no orientation. A diagnosis is a reduction in complexity – but may vary widely: e.g. precise or vague, general or specific, situational or general, dynamic or categorical, anamnestic or actual, established or flexible, behaviour- or symptom-related, with focus on the individual or on context, problem- or solution-orientated, deficit- or resource-orientated – to at least name the most important differentiations. From the perspective of our metatheory diagnosing aids the change process. Therefore, only this is diagnosed and not the client. Thus, it requires a concept for the process diagnosis. This must come out of the eight guiding distinctions and their manifestation by the client. Based upon the analysis of his decision-making premises, a picture emerges, which can change and vary from counselling session to counselling session, and from subject to subject. The more circumscribed, the more definite, the closer a counselling theory presents itself in style to scientific diagnostics, the more it risks falling into the trap of unambiguity and objectivity of diagnoses. Counselling, though, must remain a ‘management of vague issues’, as formulated by system theoretician Prof. Dr. Peter Fuchs. Otherwise its effectiveness can be considerably impeded in favour of an apparent objectivity and accountability.