The Office of the State Appellate Defender is a state agency created by the State Appellate Defender Act (725 Illinois Compiled Statutes, Act 105). The principal function of the Office of the State Appellate Defender is to represent indigent persons on appeal in criminal cases when appointed by the Illinois Supreme Court, the Appellate Court or the Circuit Court.

To provide for this legal representation, the State Appellate Defender has district offices in each of the five appellate court districts of Illinois (Chicago, Elgin, Ottawa, Springfield and Mt. Vernon). The Capital Post-Conviction Unit, with offices in Springfield and Chicago, represents death row inmates in post-conviction proceedings. Although the death penalty has been abolished and death sentences commuted, the Capital Post-Conviction Unit will remain in operation to complete its representation of former capital clients in their post-conviction proceedings.

The Office of the State Appellate Defender is nationally recognized for excellence in providing indigent criminal services. In 1986, OSAD was honored by the American Bar Association and the National Legal Aid and Defender Association as the outstanding Public Defender office in the nation.