The author discusses the legal regulations for the appointment of forensic-psychological experts and thereby defines their procedural competencies. He then analyses the concept of special knowledge in the area of psychology. This set of knowledge and skills is different from lay psychological knowledge which is not "special knowledge" in the strict sense. The main body of the article is devoted to the discussion of the theoretical and methodological foundations on which expert forensic-psychological opinions should be based. Finally, the author discusses 11 diagnostic-research problems delineating specific areas of forensic-psychological expertise which constitute expertal assignments for forensic-psychological experts.