Evidence in the form of an expert opinion is usually of key importance for settling a pending case in any type of proceedings. In some cases, the role of the expert witness is closer to that of a judge rather than that of a witness, since a judge who does not have special knowledge often has to use evidence given by an expert to render a judgement. For this reason, issuing a false expert opinion results in a very high risk of delivering a wrong and unfair decision in a given case, which in turn has a negative impact on the social perception of the functioning of the justice system. In the Polish Criminal Code, criminal responsibility for issuing a false opinion is stipulated in Article 233 (4) and (4a) of the Penal Code. At the same time, despite a very large number of reports of suspicion that a crime has been committed by an expert witness, only a negligible number of investigations result in a bill of indictment and a conviction, which causes virtual impunity of perpetrators and has a negative impact on the functioning of criminal justice. Due to the diagnosed research gap in this area, the need to investigate and describe the phenomenon of issuing false opinions by expert witnesses, both in normative and criminological terms, on the basis of empirical research, has been clearly seen. The main objective of the research has been to characterise the phenomenon in question on many levels and to determine its real extent, its etiology and symptomatology. An additional aim of the research has been the verifi cation of research hypotheses and recognition of the normative sphere of the expert witness’s status, expert evidence, and principles of responsibility for issuing false opinions. The research fi ndings have resulted in proposals of solutions aimed both at limiting the phenomenon of issuing false opinions and more effective prosecution of perpetrators of crimes under Article 233 (4) of the Penal Code, which in turn may translate into more effi cient functioning of the entire justice system, as expert witnesses and their work are an extremely important aspect of thereof. The conducted research has fully confi rmed the research hypotheses and precisely indicated defective areas of expert evidence, and consequently the need to introduce immediate legislative changes. Some of the research conclusions and de lege ferenda postulates were implemented into the amended provisions of the Penal Code in 2016, which fully confi rms their legitimacy. Unfortunately, there is still no legal act of statutory rank which would comprehensively regulate the status of expert witnesses and expert evidence.