This master thesis deals with the role of the court expert in pre-trial and criminal proceedings. With the rapid development of science and technological advances, judges often find themselves dealing with cases where legal science cannot provide a fair and justified judicial decision without the assistance of an expert. As a result, law and other sciences collide. In the first part of the thesis, the author presents the general characteristics of expert opinion in Slovenian criminal law, and explains how it is regulated different legal systems. It describes the advantages and disadvantages of a particular regulation, and in the end of the chapter connects that to the problems of the regulation in the Slovenian legal system. The author also proposes the solutions for system optimisation. In the next part, the author makes clear his position on the possibilities of ordering a pre-trial expert opinion with the assistance of the investigating magistrate and the public prosecutor. The central part of the thesis deals with the presentation of expert assessment. The question that arises is if and to what extent the judges should consider the expert opinion from the field from which they lack knowledge. In regards to that, the author outlines the guidelines and methods that judges should follow when assessing the work of experts. This is then followed by a presentation of the use of expert opinion by the defence as a tool for successfully opposing the evidence with an expert. Both the defence and the judge are faced with the problem of understanding the complex scientific fields of the subject matter and therefore really need the assistance of a court expert when trying to call into question the expert opinion. The thesis concludes with final thoughts that summarise the content that has been presented and contains ideas as well as suggestions for optimising the system of expert opinion in the Slovenian criminal justice system.