The Illinois Natural Areas Inventory program is administered by the Illinois Department of Natural Resources Division of Natural Heritage.  It is a separate program from the Illinois Nature Preserves Commission, and INAI sites do not automatically become Nature Preserves. However INAI work can inform the selection and designation of Illinois Nature Preserves.

The Illinois Natural Areas Inventory (INAI) provides a set of information about high quality natural areas, habitats of endangered species, and other significant natural features. Information from the INAI is used to guide and support land acquisition and protection programs by all levels of government as well as by private landowners and conservation organizations. 

The original inventory was conducted 1975-78, and as of 1978 included 1,085 significant natural areas.  Some sites were removed and others added after 1978, and as of 2002 there were 1099 sites listed, according to a description of INAI data accessed via the Illinois Geospatial Data Clearinghouse (formerly available at http://www.isgs.uiuc.edu/nsdihome/outmeta/inai.f.html). From 2007-2010, a comprehensive update of the inventory was performed.  Scientists from the Illinois Natural History Survey made substantial contributions to both the original INAI and the 2007-2010 update. 

Nominated sites from the recent INAI update will be reviewed by the Natural Areas Evaluation Committee, and if approved will be added to the list of high quality natural areas in the INAI.  INAI data are maintained in the Illinois Department of Natural Resources' Illinois Natural Heritage Database (INHD); access and use is subject to the terms of a Data License Agreement (DLA) or Memorandum of Understanding (MOU) developed between the Natural Heritage Database Program and the information requestor.  In June 2024, a list of INAI sites by county was released by the Illinois Department of Natural Resources.