Gov. Pat Quinn signed the bill into law Thursday at a new University of Chicago medical facility.
Illinois' law takes effect Jan. 1, but it'll take several months before medical marijuana will be available for purchase. The measure outlines a four-year pilot program for patients suffering from more than 30 serious illnesses or diseases.
Quinn says he's heard compelling stories from seriously ill patients - including veterans - and says medical marijuana will provide many people relief.
He also says Illinois' new law also has some of the toughest standards in the country, including background checks for all staff at state-run dispensaries and 24-hour surveillance at growing centers.
Nineteen other states and Washington, D.C., allow medical marijuana.
The bill is HB1.