Depending on the industry that your business is in, you may be required to register with the Illinois Department of Financial and Professional Regulation (IDFPR). Certain businesses need to have a license to operate in Illinois. Industries that need to register with IDFPR include banks, financial institutions, real estate, and other types of professionals. In addition to being licensed by IDFPR, the business and its employees must follow IDFPR regulations that apply to that industry.
Illinois has an invaluable webpage dedicated to connecting businesses to agencies that businesses may or are required to register with to operate in Illinois. Illinois' business hub is located at https://www2.illinois.gov/business/registration-licenses-permits