Admissibility of expert opinion is discussed in terms of the necessity of such opinion, assistance to the jury standard, admissibility under the Federal Rules of Evidence, and discretion of the court. Covered under limitations are such procedural limitations as timeliness and relevancy, which requires that testimony cannot be unduly prejudicial or cumulative. Cross-examination of the expert is discussed as a limitation on the impact of the testimony and related to preparation and goals of the cross-examination. Ultimate facts in the issue are also considered as limitation. To explain the need for expert opinion, the discussion reviews the necessity of expert opinion in criminal cases and its essential function in the cause of action. A total of 80 footnotes are provided.