Bolstering means to build up or support. Bolstering testimony is generally improper. Bolstering testimony is improper when it relates to the witness’s truthfulness on a specific occasion and when the foundational requirements of evidentiary rules are not met. Although bolstering testimony is normally inadmissible, the opposing party may "open the door" for such testimony. For example, where the defense on cross-examination creates the impression that an investigation was less than thorough, the defense has opened the door and the prosecution should be allowed the opportunity to dispel that impression.
Federal Rule of Evidence Rule 608(b), reads in relevant part as follows:
(a) Opinion and reputation evidence of character. The credibility of a witness may be attacked or supported by evidence in the form of opinion or reputation, but subject to these limitations: (1) the evidence may refer only to character for truthfulness or untruthfulness, and (2) evidence of truthful character is admissible only after the character of the witness for truthfulness has been attacked by opinion or reputation evidence or otherwise.
(b) Specific instances of conduct. Specific instances of the conduct of a witness, for the purpose of attacking or supporting the witness' credibility, other than conviction of crime as provided in rule 609, may not be proved by extrinsic evidence. They may, however, in the discretion of the court, if probative of truthfulness or untruthfulness, be inquired into on cross-examination of the witness (1) concerning the witness' character for truthfulness or untruthfulness, or (2) concerning the character for truthfulness or untruthfulness of another witness as to which character the witness being cross-examined has testified.
Under Rule 608(b), attacks or rehabilitation of witnesses with respect to credibility are limited to reputation and opinion evidence. Specific instances of the conduct of a witness for the purpose of attacking or supporting the witness's credibility generally are barred.