In Barber v. Page, 1967, the U.S. Supreme Court ruled that a defendant's Sixth Amendment right to confront witnesses against him was violated when the prosecution introduced at trial prior recorded testimony of a witness who had been incarcerated in another state and thus was not available for cross-examination by defense counsel. The court held that unless it is shown that the witness is unavailable and that his absence has been adequately explained to the jury, such evidence cannot be admitted into court proceedings because it denies defendants their constitutional rights under Confrontation Clause of Sixth Amendment which guarantees them an opportunity to challenge credibility of witnesses through cross-examination.

In the dissenting opinion for Barber v. Page, Justice Harlan argued that the majority's decision to exclude testimonial evidence from a preliminary hearing violated established legal principles and could potentially undermine effective law enforcement. He contended that there was no constitutional requirement for confrontation at every stage of criminal proceedings, especially when it comes to hearsay exceptions like former testimony. Furthermore, he disagreed with the majority's interpretation of "unavailability" in relation to witnesses; he believed it should not be limited solely to death or illness but also include situations where a witness is outside court jurisdiction and thus cannot be compelled to testify. In his view, this broader understanding would better serve justice by allowing relevant evidence while still protecting defendants' rights through cross-examination during initial testimony.