A U.S. Supreme Court case in which the Court ruled that the Sixth Amendment right to an attorney is a fundamental right that's essential to a fair criminal trial. States are required by the 14th Amendment due process clause to respect this right. Because of Gideon, a state criminal defendant who can't afford an attorney is entitled to the appointment of legal counsel at no expense.
Later, in Alabama v. Shelton, 535 U.S. 634 (2002), the Supreme Court clarified that counsel must be appointed for a state criminal defendant who can't afford to pay for a lawyer in any state law criminal case where jail time is possible.
Full text: Gideon v. Wainright