The defense, under the Sixth Amendment, must have an opportunity to "confront" and cross-examine witnesses. The Confrontation Clause relates to the common law rule preventing the admission of hearsay, that is to say, testimony by one witness as to the statements and observations of another person for the purpose of proving that the statement or observation was accurate. The rationale was that the defendant had no opportunity to challenge the credibility of and cross-examine the person actually making the statements. Certain exceptions to the hearsay rule have been permitted; for instance, admissions by the defendant are admissible, as are dying declarations. Nevertheless, the Supreme Court has held that the hearsay rule is not exactly the same as the confrontation clause; hearsay may, in some circumstances, be admitted though it is not covered by one of the long-recognized exceptions; for example, prior testimony may sometimes be admitted if the witness is unavailable. In the 2004 case of Crawford v. Washington, the Supreme Court increased the scope of the confrontation clause in trials. Justice Scalia's opinion made any "testimonial" out-of-court statements inadmissible if the accused did not have the opportunity to cross-examine that accuser. "Testimonial" becomes a term of art here, meaning any statements that an objectively reasonable person in the declarant's situation would have deemed likely to be used in court. The most common application of this would come after a declarant made a statement to a police officer, and then that officer testifies about that statement at trial.
The defendant must also be permitted to call witnesses in his favor. If such witnesses refuse to attend, they may be compelled to do so by the court at the request of the defendant. In some cases, however, the court may refuse to permit a defense witness to testify. If, for example, a defense lawyer fails to notify the prosecution of the identity of its witnesses in order to gain a tactical advantage, the witnesses whose identities were undisclosed may be precluded from testifying.
The right to confront and cross-examine witnesses also applies to physical evidence; the prosecution must present physical evidence to the jury, providing the defense ample opportunity to cross-examine its validity and meaning. Prosecution generally may not refer to evidence without first presenting it.