This master's thesis addresses the work of the psychiatric and psychological court appointed expert, the court's attitude towards the court appointed expert and the impact of the expert opinion on the outcome of criminal proceedings. (Too) often, judges face court appointed expert with questions beyond the legal framework; this indicates the judges' indecision when in distress and them turning to the court appointed expert, expecting them to answer the questions that should have been formulated by the judges themselves. The thesis thus draws attention to the importance of distinguishing between the function of court appointed expert and the function of judge, since in many cases judges rely wholly on the expert's opinion and, in the grounds, do not make their own evidentiary assessment but merely transcribe part of the expert opinion. The findings of the research part of this thesis, which address the impact of the expert opinion on the outcome of the criminal proceedings, are based on an insight into the selected court files at the Ljubljana District Court. The review of criminal cases has shown that the influence of the psychiatric and psychological court appointed expert on the outcome of the criminal proceedings is considerable, which raises the question of whether the defendant is truly afforded fair trial or not.