Most evidence is offered by witnesses who testify before the court. Here, the question of the witness's personal competency must be resolved; it must be shown that the witness was able to know, understand, and remember the matters on which he or she is to be examined. Thus, a witness must possess the sensory faculties needed to apprehend the facts reported and must not be considered mentally ill or incompetent. Children offered as witnesses are examined by the judge to determine their intelligence and understanding.
The witness is first directly examined by the party who offers him or her, then is cross-examined by the adversary. No witness may express an opinion on any matter when the jury can draw its own conclusions from the facts; but on technical questions an expert witness (e.g., a physician) may state an opinion. Hearsay declarations (e.g., testimony concerning a statement made out of court by a person not now before the court) usually are excluded on the grounds that the person who made the statement is not available for cross-examination or for evaluation by the judge or jury. Only when the circumstances of the statement afford a high probability of its truth may it be admitted.
A witness may be excused from testifying about certain matters if he or she pleads personal privilege. In general, information confided in the course of the relations of attorney and client, priest and penitent, physician and patient, and husband and wife is subject to this privilege. In some jurisdictions such witnesses are incompetent to testify (cannot testify). Witnesses are further protected by the Fifth Amendment privilege of withholding evidence that might be self-incriminating. Criminal defendants have the privilege of refusing to take the witness stand (in which case the jury may make no negative assumptions concerning the reasons for such a refusal) and, in most situations, evidence of previous criminal convictions is inadmissible. Under the common law, parties to a civil suit and the defendant in a criminal action were not permitted to testify, but these rules have been abandoned.