The list of ecoregions in Illinois are lists of terrestrial ecoregions (see also, ecosystem) of the United States' State of Illinois, as defined separately by the United States Environmental Protection Agency (USEPA), and by the World Wildlife Fund. Illinois' ecology is in a land area of ; the state is long and wide and is located between latitude: 36.9540ð to 42.4951ð N, and longitude: 87.3840ð to 91.4244ð W, with primarily a humid continental climate.
The EPA ecoregion classification system has four levels, but only Levels I, III, and IV are shown on this list. Level I divides North America into 15 broad ecoregions (or biomes). Illinois is almost entirely within the Eastern Temperate Forest environment Level I region, although very small sections in its extreme west are in the Great Plains, Level I region. Level IV ecoregions (denoted by numbers and letters) are a further subdivision of Level III ecoregions (denoted by numbers alone). In general, Illinois transitions from the forests, to savannah, to tall grass prairie, and is now largely used for agriculture or urbanized, although in its far south are the forested highlands of the Shawnee Hills and along its major rivers varying topography and biome occurs. Its larger ecoregion areas are 'corn belt' plains, known for rich, thick loess (in its north, center and east, particularly 54a) and the 'rivers and hills' region, which also has large till plains in Illinois' south (72j) and west (72i).
These forests stretch from eastern Texas and northern Florida to the Adirondacks and Wisconsin. For a general description of these forests, refer to Temperate Deciduous Forest. The standard reference is The Deciduous Forest of Eastern North America.