The agencies of Illinois's government publish information across a wide variety of subjects, from animal husbandry (Illinois Department of Agriculture) through marriage statistics and environmental cleanup sites to zoology (coincidentally, also Illinois Department of Agriculture). If you are researching the implications of an issue at the county or local level, Illinois government information might be a good place to turn. County and local statistics can also be more available from Illinois publications than from federal publications, with the major exception of the U.S. Census. Also, some topics are covered differently by state governments than by the federal government. A good example of this is education finance, which is worked out in detail by state boards of education rather than by the U.S. Department of Education.
Like many governments, Illinois is publishing more and more online, and even more online documents are available via IGI (Illinois Government Information). Agency web sites also have lists of publications although older publications are sometimes taken down to make way for newer ones. And many publications are still being printed and shipped to libraries.
The Illinois Documents Depository Program began in the late 1960's, although the University of Illinois Library obtained many state documents prior to becoming a depository library. In particular, legislative documents from the time Illinois became a state in 1818 are in the Main Stacks. All Illinois documents in the library's collection are represented in the online catalog. Some are not represented at the level of each individual item, like the daily transcripts of the floor sessions for the Illinois General Assembly. These transcripts are in microfiche format from the time that they began to be transcribed in the 1970s through the late 1990s, when the General Assembly began posting transcripts online.
Illinois documents in print format have Dewey Decimal call numbers and are shelved among the non-depository books in libraries across campus, although the largest concentration is in the Main Stacks. Some Illinois documents are indexes or reference sources that are kept in the Reading Room of the Main Library. The online catalog is a good way to find Illinois documents in print and microfiche, as well as the URLs for many online state publications. If you are searching for Illinois government documents specifically, use the catalog's advanced search screen to search for Illinois as an author word along with whatever title or subject terms you want to search for.