The Sixth Amendment to the United States Constitution states: "In all criminal prosecutions, the accused shall enjoy the right . . . to have the Assistance of Counsel for his defence." The statutory right to counsel in federal proceedings was reaffirmed by the U.S. Supreme Court in the 1938 decision Johnson v. Zerbst.
In Powell v. Alabama, the Court held that counsel was required in all state capital proceedings.
The most well known opinion on the right to counsel is the Supreme Court's decision Gideon v. Wainwright. The Court unanimously held that an indigent person accused of a serious crime was entitled to the appointment of defense counsel at state expense.
In federal court, the provision of counsel is governed by statute, specifically 18 U.S.C. sec. 3006A. For more information, see Appointing Counsel.