Unlawful evidence in the Croatian legal system first saw the light of the day in Article 29 of the Constitution of the Republic of Croatia and was formulated as illegally obtained evidence. Modeled on the modern approach to the institute of unlawful evidence of the European Court of Human Rights, the legislator formulated Article 220.a of the Civil Procedure Act. The concept of unlawful evidence is taken from the Constitution of the Republic of Croatia. The court can allow the use of unlawful evidence and take into account its content, in order to establish a decisive fact. In doing so, he must apply the test of proportionality between the violation of constitutional or conventional rights for the purpose of using unlawful evidence and the interest in properly and completely establishing the factual statement in the proceedings. The legal amendment in question is good and desirable, but the criticisms of relevant legal experts should be taken into account in relation to its vagueness and deficiency. In order to understand the concept and essence of the institute in question, the thesis describes the development of access to unlawful evidence in the field of criminal procedural law, which was marked by the significant intervention of the Constitutional Court of the Republic of Croatia. Many representatives of the procedural doctrine consider that the current Article 10 of the Criminal Procedure Act should have served as a model for the regulation of unlawful evidence in a form adapted to the specifics of civil proceedings. After a theoretical approach to the institute of unlawful evidence, the thesis deals with the relevant practice of the Supreme and Constitutional Courts of the Republic of Croatia and the practice of the European Court of Human Rights, which is particularly important for understanding the principle of a fair trial, which must be satisfied when making a decision on the presentation of a particular piece of evidence. If the court decides to introduce unlawful evidence by applying the proportionality test, the decision to introduce such evidence should be justified and supported by valid arguments.