Psychological and psychiatric examination is an extremely important means of evidence, especially in the kind of lawsuits that relate to human psyche. Thanks to their competence, an expert is able to determine not only the presence of some kind of mental illness or personality disorder in a person, but also to describe the dynamics of the mental processes and their severity. In the context of ecclesiastical cases to declare the nullity of marriage it is extremely important, since only a serious psychological anomaly in the individual entering a marriage has the potentially destructive impact on the validity of consensus, making the person unable to express it. Expertise is not a judgment on the case and will not eventually determine whether the marriage is to be declared null. Its critical evaluation and determination of the value of evidence lies with the judge. The expert is only to assist the judge in reaching the truth in the process, and the judge issues a conclusion, assessing all the evidence, including the opinion given by the expert. The principle of free interpretation on the part of the judge remains fundamental, as it gives him the dominant role in the assessment of the value the expert opinion may have in creating a moral certainty necessary for a fair judgment. Although the judge does not have to be guided by the expert conclusions, due to their significant value in dealing with issues arising from mental incapacity, it would be inadvisable, if that opinion has adequate factual and argumentative basis, and is not in contradiction with the Christian vision of an individual and marriage.