(a-5) No window treatment or tinting shall be applied to
the windows immediately adjacent to each side of the driver,
except:
(1) on vehicles where none of the windows to the rear
of the driver's seat are treated in a manner that allows
less than 30% light transmittance, a nonreflective tinted
film that allows at least 50% light transmittance, with a
5% variance observed by any law enforcement official
metering the light transmittance, may be used on the side
windows immediately adjacent to each side of the driver.
(2) on vehicles where none of the windows to the rear
of the driver's seat are treated in a manner that allows
less than 35% light transmittance, a nonreflective tinted
film that allows at least 35% light transmittance, with a
5% variance observed by any law enforcement official
metering the light transmittance, may be used on the side
windows immediately adjacent to each side of the driver.
(3) on multipurpose passenger vehicles, as defined by
Section 1-148.3b of this Code, a nonreflective tinted film
originally applied by the manufacturer, that allows at
least 50% light transmittance, with a 5% variance observed
by any law enforcement official metering the light
transmittance, may be used on the side windows immediately
adjacent to each side of the driver.
(b) On motor vehicles where window treatment has not been
applied to the windows immediately adjacent to each side of the
driver, Nothing contained in this Section shall prohibit the
use of a nonreflective, smoked or tinted glass, nonreflective
film, perforated window screen or other decorative window
application on windows to the rear of the driver's seat shall
be allowed, except that any motor vehicle with a window to the
rear of the driver's seat treated in this manner shall be
equipped with a side mirror on each side of the motor vehicle
which are in conformance with Section 12-502.

UPDATE: 7-28-9. Andrea at the Illinois Capitol Office answers a very good question from one of our readers.

Question: I emailed my alderman's office, and they said that even if Illinois passes a law allowing tint, the Chicago Municipal Code will supercede that law. So tint will still be illegal with a $250 ticket in Chicago until the Municipal Code is changed (at least according to my alderman's office).

Answer: NO that is not true. The Illinois Vehicle Code (Chapter 11 specifically) allows Chicago to adopt ordinances in regards to the movement of vehicles and rules of the road. However, tinting is not a moving ordinance but an equipment ordinance (Chapter 12 of the Illinois Vehicle Code) thus Chicago is not exempt from the law. This information came from the Illinois State Police.