The Diocese of Springfield in Illinois is a Latin Church ecclesiastical territory, or diocese, of the Catholic Church in south central Illinois in the United States. The mother church is the Cathedral of the Immaculate Conception in Springfield.

The Archdiocese of Chicago is a Latin Church ecclesiastical jurisdiction, an archdiocese of the Catholic Church located in Northeastern Illinois, in the United States. It was established as a diocese in 1843 and elevated to an archdiocese in 1880. It serves the more than 2 million Catholics in Cook and Lake counties in the state of Illinois, an area of 1,411 square miles (3,650 km2). The archdiocese is divided into six vicariates and 31 deaneries.