The criteria for assessment of legal capacity to stand trial have long been a significant issue in the Polish criminal law. The main problem in opinionating practice is the fact that the code of penal procedure and the executive penal code do not provide any univocal criteria of a mental disease that should be met according to the legal regulations in force. Because of their nature and lack of uniformity, depression and affective disorders pose a particularly great problem for experts who should opinionate on the legal capacity of the suspected and the accused to stand trial. On the one hand, it cannot be accepted that a mental illness renders a person unable to substantially understand legal proceedings and to make a rational defense. On the other hand, however, emotional reactions and mild-degree depression disorders that are only natural when an individual violates law should not be allowed to be regarded as a grave disease and to paralyze legal proceedings. In the present study, the authors have attempted to describe the guidelines that should be followed by court-appointed experts in psychiatry that are commissioned by the court to assess the mental state of an individual. The report emphasizes that in such cases, a thorough analysis, based on medical and legal premises and the experience of the involved court experts are necessary.