The Illinois Senate is the upper chamber of the Illinois General Assembly, the legislative branch of the government of the state of Illinois in the United States. The body was created by the first state constitution adopted in 1818. The Illinois Senate is made up of 59 senators elected from individual legislative districts determined by population. Under the Illinois Constitution of 1970, senators are divided into three groups, each group having a two-year term at a different part of the decade between censuses, with the rest of the decade being taken up by two four-year terms. Depending on the election year, roughly one-third, two-thirds, or all Senate seats may be contested. In contrast, the Illinois House of Representatives is made up of 118 members with its entire membership elected to two-year terms. House districts are formed by dividing each Senate district in half.

The Illinois Senate convenes at the Illinois State Capitol in Springfield, Illinois. Its first official working day is the second Wednesday of January each year. Its primary duties are to pass bills into law, approve the state budget, confirm appointments to state departments and agencies, act on federal constitutional amendments and propose constitutional amendments for Illinois. It also has the power to override gubernatorial vetoes through a three-fifths majority vote. The Illinois Senate tries impeachments made by the House of Representatives, and can convict impeached officers by a two-thirds vote.

Illinois (ⁱ/ˌɪlɨˈnɔɪ/ IL-i-NOY) is the 25th most extensive and the 5th most populous of the 50 United States, and is often noted as a microcosm of the entire country. With Chicago in the northeast, small industrial cities and great agricultural productivity in central and northern Illinois, and natural resources like coal, timber, and petroleum in the south, Illinois has a broad economic base. Illinois is a major transportation hub. The Port of Chicago connects the state to other global ports from the Great Lakes, via the Saint Lawrence Seaway, to the Atlantic Ocean; as well as the Great Lakes to the Mississippi River, via the Illinois River. For decades, O'Hare International Airport has ranked as one of the world's busiest airports. Illinois has long had a reputation as a bellwether both in social and cultural terms and politics.

Although the state's largest population centers today are in northern Illinois, originally the state's population grew from south to north, with settlers arriving from Kentucky in the 1810s. In 1818, Illinois achieved statehood. Chicago was founded in the 1830s on the banks of the Chicago River, one of the few natural harbors on southern Lake Michigan. Railroads and John Deere's invention of the self-scouring steel plow turned Illinois' rich prairie into some of the world's most productive and valuable farmlands, attracting immigrant farmers from Germany and Sweden. By 1900, the growth of industrial jobs in the northern cities and coal mining in the central and southern areas attracted immigrants from Eastern and Southern Europe. Illinois was an important manufacturing center during both world wars. The Great Migration established a large community of African Americans in Chicago that created the city's famous jazz and blues cultures.

Mike Bost (born December 30, 1960) is a Republican member of the Illinois House of Representatives, representing the 115th district where he has served since 1995.

Bost was a firefighter before entering politics and becoming a full-time state legislator and business owner. He graduated from Murphysboro High School, attended the University of Illinois Certified Firefighter II Academy, and served in the United States Marine Corps (1979–82). He and his wife Tracy have three children and six grandchildren.

Bost sponsors many firearm bills in support of the right to bear arms. He was a passionate opponent of former Illinois governor Rod Blagojevich. In May 2012, upset over a bill to overhaul state pensions, Bost screamed and threw papers on the Illinois House floor because the House was only given twenty minutes to review and vote on a two-hundred page bill.