The article considers some crucial aspects of the use of expert witness opinion as means of introducing materials from covert operations performer by the appropriate security authorities and law enforcement as evidence in formal criminal proceedings. The most important is the issue of formally including information which originated from covert operations into the legal proceedings. Traditional perspective forbids the use of evidence from “uknwnown origin” - such as covert operations, which nature and cannot be thoroughly revealed. Presenting an expert’s opinion on such information of evidentiary value may serve as a method of introducing such materials into the criminal proceedings.