Each state elects two senators to serve for six years, and members of the House to two-year terms.
Before becoming a state, the Illinois Territory elected a non-voting delegate at-large to Congress from 1812 to 1818.
Starting on December 3, 1812, Illinois Territory sent a non-voting delegate to the House.
Following the 1830 census, Illinois was apportioned three seats, all elected via single-member districts.
Following the 1870 census, Illinois was apportioned 19 seats, all elected via single member districts.
Following the 1950 census, Illinois was apportioned 25 seats, all of which were elected from single-member districts.