Eliot Ness (April 19, 1903 – May 16, 1957) was an American Prohibition agent known for his efforts to bring down Al Capone while enforcing Prohibition in Chicago. He was leader of a team of law enforcement agents nicknamed The Untouchables, handpicked for their incorruptibility. The release of his memoir The Untouchables, months after his death, launched several screen portrayals establishing a posthumous fame for Ness as an incorruptible crime fighter.

Eliot Ness was born on April 19, 1903, in the Roseland neighborhood of Chicago, Illinois. He was the youngest of five children born to Peter Ness (1850–1931) and Emma King (1863–1937). His parents, both Norwegian immigrants, operated a bakery. Ness attended Christian Fenger High School in Chicago. He was educated at the University of Chicago, graduating in 1925 with a degree in political science and business administration, and was a member of Sigma Alpha Epsilon. He began his career as an investigator for the Retail Credit Company of Atlanta assigned to the Chicago territory, where he conducted background investigations for the purpose of credit information. In 1929, he returned to the university to take a graduate course in criminology taught by August Vollmer, a noted police reformer and chief of the Berkeley Police Department. Vollmer's ideas about professionalizing law enforcement would influence Ness throughout his career.

Ness's brother-in-law, Alexander Jamie, an agent of the Bureau of Investigation (which became the Federal Bureau of Investigation in 1935), influenced Ness to enter law enforcement. Ness joined the U.S. Treasury Department in 1926, working with the 1,000-strong Bureau of Prohibition in Chicago.

In March 1930, attorney Frank J. Loesch of the Chicago Crime Commission asked President Herbert Hoover to take down Al Capone. Agents of the Bureau of Internal Revenue, working under Elmer Irey and Special Agent Frank J. Wilson of the Intelligence Unit, were already investigating Capone and his associates for income tax evasion. In late 1930, Attorney General William D. Mitchell, seeking a faster end to the case, implemented a plan devised by President Hoover for sending a small team of Prohibition agents, working under a special United States attorney, to target the illegal breweries and supply routes of Capone while gathering evidence of conspiracy to violate the National Prohibition Act (informally known as the Volstead Act). U.S. attorney George E.Q. Johnson, the Chicago prosecutor directly in charge of both the Prohibition and income tax investigations of Capone, chose the 27-year-old Ness (now assigned to the Justice Department) to lead this small squad.

With corruption of Chicago's law enforcement agents endemic, Ness went through the records of all Prohibition agents to create a reliable team (initially of six, eventually growing to about ten) later known as "The Untouchables." Raids against illegal stills and breweries began in March 1931. Within six months, Ness's agents had destroyed bootlegging operations worth an estimated $500,000 (almost $9.9 million in 2022) and representing an additional $2 million ($39.5 million in 2022) in lost income for Capone; their raids would ultimately cost Capone in excess of $9 million ($178 million in 2022) in lost revenue. The main source of information for the raids was an extensive wiretapping operation.

In 1931, a member of Al Capone's gang promised Ness that he would receive $2,000 every week ($36,684.27 in 2022) if he ignored their bootlegging activities. Ness refused the bribe. Failed attempts by members of the Chicago Outfit to bribe or intimidate Ness and his agents inspired Charles Schwarz of the Chicago Daily News to begin calling them "untouchables". George Johnson adopted the nickname and promoted it to the press, establishing it as the squad's unofficial title.

The efforts of Ness and his team inflicted major financial damage on Capone's operations and led to his indictment on 5,000 violations of the Volstead Act in June 1931. Federal judge James H. Wilkerson prevented that indictment from coming to trial, instead pursuing the tax evasion case built by George Johnson and Frank Wilson. On October 17, 1931, Capone was convicted on three of 22 counts of tax evasion. He was sentenced to eleven years in prison and, following a failed appeal, began his sentence in 1932. On May 3, 1932, Ness was among the federal agents who took Capone from the Cook County Jail to Dearborn Station, where he boarded the Dixie Flyer to the Atlanta Federal Penitentiary—the only time the two men are known to have met in person.

In 1932, Ness was promoted to Chief Investigator of the Prohibition Bureau for Chicago. Following the end of Prohibition in 1933, he was assigned as an alcohol tax agent in the "Moonshine Mountains" of southern Ohio, Kentucky, and Tennessee, and in 1934 he was transferred to Cleveland, Ohio. In December 1935, Cleveland mayor Harold H. Burton hired Ness as the city's Safety Director, which put him in charge of both the police and fire departments. Ness soon began a reform program inspired by the ideas of August Vollmer, which focused on professionalizing and modernizing the police, stopping juvenile delinquency, and improving traffic safety. He declared war on the mob, and his primary targets included "Big" Angelo Lonardo, "Little" Angelo Scirrca, Moe Dalitz, John Angerola, George Angersola, and Charles Pollizi.