Witness impeachment is the process of casting doubt on the credibility of a person who is giving testimony in a trial.
Under the United States' Federal Rules of Evidence, (F.R.E. 607), any party may attack the credibility of any witness.
The credibility of a witness can be called into question by introducing evidence as to his bias either in favor or against any party.
The reliability of a witness can also be eroded by exposing his mutually contradictory statements.
A witness can also be impeached by evidence of his disreputable character for dishonesty.
Exposure of prior criminal acts can also discredit a witness. Prior conviction of the witness for any criminal act of dishonesty is also admissible as evidence. If the witness is the criminal defendant a felony conviction with at least one year of imprisonment is admissible in evidence, if according to the judge its probative value exceeds the potential for prejudice. In respect of any witness other than the defendant in criminal case, a felony conviction is also admissible. State felonies where the witness was set free at least ten years ago or found guilty minimum ten years ago are also admissible if the probative value outweighs prejudice.