A presentation of the institution of an expert and quasi-expert in criminal proceedings and comments on the products of their work, i.e. trial and out-of-trial opinion, also known as a private opinion, with particular attention to the changes introduced by the amendment to the Code of Criminal Procedure of July 1 2015. It also presents the institution of a quasi-expert (private expert), educated outside the criminal trial, and the nature of the private opinion he prepares, including the issue of admissibility of a private opinion and its assessment. The proposed changes mentioned in the literature on the subject, as well as my own proposals for changes aimed at increasing the control over the reliability and correctness of expert opinions and enabling wider use of private opinions in criminal proceedings, were indicated and discussed.