In the article analysed the practice of obtaining opinions of expert psychologist in juvenile delinquency cases. The choice of types of cases is premeditated, and is based on their specific character. Juvenile delinquency cases are special in terms of their subjects (children and adolescence) and aims to be achieved (the welfare of the child/adolescence). Forensic psychology expertise plays a significant part in arriving at a court ruling. Therefore, it is important to raise the quality of diagnostic procedures, expertise activities, and to establish evaluation standards for evidence from psychological expertise. The presentation of proposed psychological expertise standards should take a form of guidelines and recommendations to be met by the experts, and serve as an aid to expertise evaluation performed by courts. To reach these goals, we gathered and analyzed court records of juvenile cases in six districts (N = 253). The results of the research are related to a) the analysis of the methodological and diagnostic procedures used by experts in the process of psychological evaluation in juvenile cases and b) the formulation of principles for evaluating the evidence of psychological opinions for trial authorities. The research project indicates the practice of psychological experts by the court, the diagnostic procedure and the method of formulating psychological opinions. The analysis of the material showed, first of all, the diversity of the diagnostic and opinion practices of psychologists, thus confirming the lack of procedures standardizing the process of psychological evaluation. The variety of assessment tools, and method, and areas of diagnosis make difficulties in assessing of the evidentiary value of psychological-court opinions. The lack of principle for assessing level of opinions’ quality may promote the practice of so-called junk science.