Crawford v. Washington radically transformed the doctrine governing the Confrontation Clause of the Sixth Amendment to the Constitution. Before Crawford, a prosecutor could introduce against an accused evidence of a hearsay statement, even one made in contemplation that it would be used in prosecution, so long as the statement fit within a "firmly rooted" hearsay exception or the court otherwise determined that the statement was sufficiently reliable to warrant admissibility. Crawford recognized that the Clause is a procedural guarantee, governing the manner in which prosecution witnesses give their testimony. Therefore, a prosecutor may not introduce a statement that is testimonial in nature to prove the truth of a matter that it asserts unless the accused has, or has had, an opportunity to be confronted with the witness who made the statement.
Friedman, Richard D. "Confrontation and Forensic Laboratory Reports, Round Four." Tex. Tech L. Rev. 45, no. 1 (2012): 51-82.