According to the federal "Endangered Species Act" of 1973, an endangered species is one that is at risk of "extinction through all or a significant portion of its range" in the United States. A threatened species is "one that is likely to become endangered" in the near future throughout all or part of its range within the United States.

As of 1998, the list of state endangered and threatened species in Illinois included 146 animals and 331 plants. The Illinois Endangered Species Protection Act, which is based upon the Federal Endangered Species Act, states that a species is endangered when at risk for extinction through all or part of its range in the state of Illinois. Likewise, a threatened species is one which is likely to become endangered in the near future.

Loggerhead Shrike chicks Midewin NTP Candidate for Federal Endangered Status Joe Milosovich