The Confrontation Clause of the 6th Amendment mandates that in all criminal prosecutions, the accused "shall enjoy the right...to be confronted with the witnesses against him." In Crawford v. Washington, the Supreme Court determined that if a hearsay statement was "testimonial" in nature, it could not be introduced at trial, in the absence of the declarant (the person who allegedly said what was said), unless the defendant had a previous chance to cross-examine the declarant.
Crawford involved a tape-recorded statement given by the defendant's wife to police describing the stabbing with which the defendant was charged. The wife did not have to testify at the trial because she was protected under the marital privilege rule.
The wife's statement was admitted at trial over objection because the trial court determined that the statement had "particularized guarantees of trustworthiness."
The Supreme Court reaffirmed the importance of the confrontation clause and drew a distinction between testimonial and nontestimonial statements.