For centuries, the common law forbade an accused person from calling witnesses in his defense in cases of treason or felony or forbade the defense witnesses, if called, to testify under oath. The English remedied that injustice for treason trials in a 1695 statute and for all cases in 1702. Sir William Blackstone in his Commentaries on the Laws of England (1765–1769) summarized the right by declaring, “[the defendant] shall have the same compulsive process to bring in his witnesses for him, as was usual to compel their appearance against him.”

Before he came to America, William Penn had been the victim of the old common law rule. In 1670, he was arrested for preaching his dissenting religious views to a group of Quakers in London, that is, to an “unlawful assembly.” Penn attempted to put on his own defense without having counsel and without the right to compel the testimony of witnesses on his behalf. The presiding judge ultimately silenced and removed Penn from the proceedings. Later in America, when Penn wrote Pennsylvania’s Charter of Privileges (1701), he included: “THAT all Criminals shall have the same Privileges of Witnesses and Council as their Prosecutors.”

After the Revolution, nine of the new state constitutions established in one form or another the right to call defense witnesses. Two of them, Massachusetts and New Hampshire, added the subpoena power. When James Madison, in the First Congress, formulated what would become the Compulsory Process Clause, he opted for including not only the right to call witnesses, but the stronger privilege of being able to subpoena them as well. Congress considered Madison’s draft language with little debate, and it became part of the Sixth Amendment without opposition. As written, the clause assures that the accused in a criminal case enjoys the right to call or subpoena witnesses, so that evidence available to the defense can be evaluated by a jury or, in a nonjury criminal case, by a judge. It became, in sum, an essential part of the right of an accused to present a defense.

Issues that surrounded the clause centered on (1) whether the right to call witnesses included the right to documentary evidence, (2) whether the right was available before as well as after indictment, and (3) whether the defense enjoyed the same degree of subpoena power as did the prosecution. Chief Justice John Marshall answered these questions when he presided over the treason trial of Aaron Burr in 1807. United States v. Burr (1807). Burr’s lawyers had requested a subpoena for documents in the possession of the president, and the government opposed the request. Marshall approved the request to obtain documents and ruled that an indictment was not needed to trigger the right to compulsory process. In upholding a 1790 federal statute that he regarded as declaratory of the constitutional right to compulsory process, Marshall also found that there was parity between the defense and the prosecution in the enjoyment of the right: “[the defense] shall have the like process of the court where he or they shall be tried, to compel his or their witnesses to appear at his or their trial as is usually granted to compel witnesses to appear on the prosecution against them.”

He further allowed the defense to obtain any relevant documents before having to decide which ones might be material at trial, and that the subpoena right allowed the defense to obtain original documents, not mere copies.

Though John Marshall had explicated the contours of the clause in the Aaron Burr trial, the Supreme Court itself had little opportunity to interpret the Compulsory Process Clause and explain its meaning prior to 1967, when the Court ruled in Washington v. Texas that the clause was so fundamental to a fair trial that it was part of the Fourteenth Amendment’s Due Process Clause and therefore binding on the states as well as on the federal government. Washington v. Texas also expanded the reach of the clause by holding unconstitutional a Texas penal statute that permitted the government to offer the testimony of one charged as a principal, accomplice, or accessory, but barred a defendant from calling the same person unless that person had been previously acquitted of the charges. The rationale for the disadvantage imposed upon defendants was that defendants would attempt to exculpate each other, and thus their testimony would be inherently biased and untrustworthy. The Supreme Court had upheld a similar rule in federal trials in United States v. Reid (1852) before changing its mind and rejecting the rule for federal trials in Rosen v. United States (1918). Although Rosen was not a constitutional ruling, the Court adopted its position in Washington v. Texas as binding under the Compulsory Process Clause, reasoning that “it could hardly be argued that a State would not violate the clause if it made all defense testimony inadmissible as a matter of procedural law.” Furthermore, the Court declared that “[i]t is difficult to see how the Constitution is any less violated by arbitrary rules that prevent whole categories of defense witnesses from testifying on the basis of a priori categories that presume them unworthy of belief.”

The Court has had few occasions since to deal with the clause. Green v. Georgia (1979) held that it was an error for a state court to exclude a codefendant’s confession offered by a defendant in a capital sentencing proceeding where the prosecution had relied on a codefendant’s confession at his own trial. In United States v. Valenzuela-Bernal (1982), the defendant complained that the government had violated his rights under the clause when it deported potential alien witnesses; the Court ruled that the defendant must show that the testimony of the deported aliens would have been favorable and material. Similarly, in Pennsylvania v. Ritchie (1987), the Court modified John Marshall’s position in the Aaron Burr case by declaring that the Compulsory Process Clause does not permit a defendant free rein to peruse files that are confidential under state law. Rather, the defendant is entitled to have access only to material evidence, a right that is in fact also guaranteed by the Due Process Clause. In Rock v. Arkansas (1987), the Court held that a per se rule excluding all hypnotically refreshed testimony impermissibly infringed on a criminal defendant’s right to call “witnesses in his own favor,” including the right to testify on one’s own behalf.

Unlike other Sixth Amendment guarantees, the right to call witnesses is at the defendant’s initiative. It is not unlimited, but subject to reasonable restrictions. Taylor v. Illinois (1988). The ordinary rules of evidence apply to the exercise of the right. The Compulsory Process Clause, for example, does not guarantee a defendant the right to use polygraph evidence in a jurisdiction that forbids such evidence. United States v. Scheffer (1998).

It should also be noted that the Supreme Court has declared that the word “witnesses” in the Compulsory Process Clause has a different meaning than the same term in the Confrontation Clause. In the Compulsory Process Clause, it refers to those who have relevant information that the defense might present at trial. In the Confrontation Clause, it refers to all witnesses who provide “testimony” against the accused, including hearsay declarants whose testimonial statements are offered by the government. Crawford v. Washington (2004).

The federal courts faced the invocation of the Compulsory Process Clause in the trial of Zacarias Moussaoui, accused of acts of terrorism in connection with the 9/11 attacks. Moussaoui requested access to al Qaeda members in United States custody in order to gain exculpatory evidence. The government wanted to deny him access to other al Qaeda members, citing national security concerns. To resolve the dispute, the Fourth Circuit balanced the interests of Moussaoui and the government. The court, as Megan Healy has written, held that “although the production of these witnesses imposed substantial burdens on the government, such burdens could not outweigh the finding that the witnesses possessed information material to the defense.” Nonetheless, in an effort to protect the government’s interest in national security, the court did not allow the defendant direct access to witnesses, but ordered the government to produce summaries of classified information compiled from interrogations of witnesses. United States v. Moussaoui, (2004).