This paper outlines the position and role of the expert witness in common law legal systems in providing evidence in criminal cases. The author also presents existing guarantees of his or her impartiality. This is an extremely interesting issue because in the adversary system unlike then in inquisitorial system litigation adversaries represent their parties case or position before an impartial judge or jury who attempt to determine the truth and pass judgment according. That’s why the influence on expert-witness opinion has got a significant impact on the process.