From www.online-springfield.com:
The first legislative session was held in the "new" state capitol in 1877. The growth of the state had increased the need for more file, storage, and office space than the "old" one allowed. Today it is the center of state government and houses the offices of the Governor, Lt. Governor, Attorney General, Secretary of State, Comptroller, and Treasurer, as well as the House of Representatives and Senate Chambers. Visitors can watch Illinois politics in action from the balcony-level seating when the legislature is in session.