In the Polish penalty procedure, the witness is an important source of evidence. The court assesses his credibility in the light of the free assessment of evidence directives’ principles (Article 7 of the Code of Criminal Procedure – CCP). In a situation where there is doubt about the mental state of the witness, his mental development condition, his ability to perceive or recreate perceptions, the court or the prosecutor, according to the content of Article 192 §2 of the CCP, may order an interrogation of awitness with the participation of, among others, expert psychologist. This may take place, for example, in case of a witness with mental disorders or awitness in old age. Opinions drawn up by an expert psychologists are also very helpful in the process of assessing the credibility of the witness’s testimony, especially in interrogation of a witness under the age of 15 (Article 185a and b of the CCP). The psychologist may also take part in interrogation of a witness in cases related to offenses specified in Article 197–199 of the CC (Article 185c § 3 of the CCP). The presence of an expert psychologist in the courtroom or in a place specifically designated by the procedural authorities, is conditioned by the disclosure of circumstances justifying the suspicion of the existence of factors that disrupt or exclude the ability to perceive, remember and reproduce observations and reporting facts. These circumstances are not always the real doubts concerning the mental state of the witness, and the role of an expert psychologist in these tasks is often not fully used.