46 athletes from Illinois marched into Seattle's Husky Stadium for the opening ceremony, which included several musical performances and a Native American dance.
More than 4,000 athletes and coaches from across the country will compete in 14 sports over the next week.
The first Special Olympics were held in Chicago 50 years ago. The games aim to promote acceptance and inclusion.
For more information on the special Olympics, visit www.specialolympics.org and for information on Special Olympics Illinois, visit www.soill.org.