Illinois ranked 5th in population in the US with an estimated total of 12,600,620 in 2002, an increase of 1.5% since 2000. Between 1990 and 2000, Illinois's population grew from 11,430,602 to 12,419,293, an increase of 8.6%. The population is projected to reach 13.4 million by 2025. Illinois ceded its 3rd-place ranking to California by 1950, and 4th place to Texas during the 1960s. In 2000, population density was 223.4 per sq mi, the 10th-highest in the US.

The population of Illinois was only 12,282 in 1810. Ten years later, the new state had 55,211 residents. The most rapid period of growth came in the mid-19th century, when heavy immigration made Illinois one of the fastest-growing areas in the world. Between 1820 and 1860, the state's population doubled every 10 years. The rate of increase slowed somewhat after 1900, especially during the 1930s, although the population more than doubled between 1900 and 1960. Population growth was very slow in the 1970s, about 0.3% a year; the rate of growth from 1980 to 1990 was a tiny 0.04%. However, a rebound occurred in the 1990s. The age distribution of the state's population in 2000 closely mirrored the national pattern, with 26.1% under age 18 and 12.1% aged 65 or older.

The rapid rise of Chicago meant that a large proportion of the state's population was concentrated in cities from a relatively early date. Thus, by 1895, 50% of Illinoisans lived in urban areas, whereas the entire country reached that point only in 1920. By 1990, 83% of the population lived in metropolitan areas, compared with 75.2% nationally. With an estimated population of 8,885,919 in 1999, Greater Chicago was the 3rd-largest metropolitan area in the nation, and alone accounted for over 70% of the total state population. The state's other major metropolitan areas, with their estimated 1999 populations, were Peoria, 346,480, and Rockford, 358,640. The largest city proper in 2002 was Chicago, with an estimated 2,886,251 residents, followed by Aurora, 156,974; Rockford, 151,068; Naperville, 135,389; Joliet, 118,423; Peoria, 112,670; and Springfield, 111,834.