The $475 million cost of the park, originally planned to open in 2000 for $150 million, grew as city planners worked with the world's best designers and architects to create a unique 21st century park, video-screen water sculptures, gardens and a $25 million stainless steel band shell by Frank Gehry covered by a latticework of steel tubes.
The centerpiece is a three-story, 110-ton polished steel sculpture called "Cloud Gate" that looks like a gleaming metal jellybean.
"Breathtaking," said Mayor Richard Daley, who will cut the ribbon before a performance by the Grant Park Orchestra Friday evening. No pop concerts will be held there until 2005.
Thousands of residents and tourists are expected to attend a $700,000 three-day party to celebrate the city's new front yard. Tours start every 30 minutes.
Private donors gave $205 million to the project, which sits atop an underground parking garage with room for 300 bicycles. There are lockers and showers. Ground was broken in August 1997.