40 ILCS 5/24-101
Notwithstanding any law to the contrary, the State of Illinois or any unit of local government or school district may enter into a written contract with any of its employees to defer a part of their gross compensation and may invest such funds in any such manner as prescribed by the deferred compensation program adopted by it under this Article. Compensation deferred pursuant to a deferred compensation program adopted under this Article shall not exceed the amount of compensation allowed to be deferred without being subject to income tax in the year in which it is earned, pursuant to Section 457 of the United States Internal Revenue Code or laws supplementary or amendatory thereto.
It is hereby declared to be in the public interest to provide public employees with a plan for the deferral of compensation and the accrual of income and gain thereon if such deferred compensation be invested, and to encourage the continued service of public employees by making available such benefits to them.
40 ILCS 5/24-101